Law of the Sea Explained
- Edmarverson A. Santos
- 1 day ago
- 14 min read
Introduction
The law of the sea answers four questions that have organized state conduct on the oceans for centuries: who may claim ocean space, and how far from shore; what the claiming state may do there, and what it must tolerate from foreign ships and aircraft; who owns the fish, the hydrocarbons, and the minerals; and whose courts decide when something goes wrong on the water. The several hundred treaty articles and the long line of judgments that make up the subject are elaborations of those questions, worked out one maritime space at a time.
One instrument now dominates the field. The United Nations Convention on the Law of the Sea (UNCLOS), adopted in 1982 and in force since 1994, binds 172 parties, including the European Union, and supplies the working vocabulary even of states that never joined it (United Nations, 1982). It is often called a constitution for the oceans, and the label is accurate in one precise sense: the Convention divides the entire ocean into maritime zones, each measured from the coast, and each carrying its own balance between coastal authority and the freedom of navigation.
That zonal logic is the key to the whole subject. Every maritime dispute begins with a location question, because the applicable rules change at each invisible line on the water. The zones themselves, though, are compromises with long histories, and they cannot be understood apart from the argument that produced them.
1. How the Law of the Sea Took Shape
1.1 Mare Liberum and the cannon-shot rule
The founding dispute was commercial before it was legal. In Mare Liberum (1609), Hugo Grotius argued that the sea could not be occupied and therefore could not be owned, a position written in service of Dutch access to the East Indies trade. John Selden answered for England in Mare Clausum (1635) that the sea could be appropriated like land. Grotius prevailed, less because his logic was stronger than because the leading maritime powers came to profit from open oceans (Tanaka, 2019).
By the eighteenth century, a compromise associated with Cornelius van Bynkershoek had settled into custom: a coastal state was sovereign over a belt of sea within the reach of its shore artillery, conventionally three nautical miles, and the water beyond was free to all. That settlement held for as long as the ocean's main value lay in navigation.
1.2 The Truman Proclamation and the Geneva Conventions
It broke when technology made the seabed and the fish worth claiming. A 1930 codification conference at The Hague could not even agree on the breadth of the territorial sea. Then, on 28 September 1945, the United States proclaimed jurisdiction and control over the natural resources of its continental shelf (United States, 1945). The claim was unilateral; it drew no protest, and it became the founding precedent of modern maritime expansion. Chile, Ecuador, and Peru responded in the 1952 Santiago Declaration with 200-mile claims of their own, directed at distant-water fishing and whaling fleets.
The first UN Conference on the Law of the Sea produced four Geneva Conventions in 1958, covering the territorial sea and contiguous zone, the high seas, the continental shelf, and high-seas fishing. Much of this codified existing custom. The central question, how wide the territorial sea could be, was left open; a second conference in 1960 failed to close the gap by a single vote. The 1958 regime also defined the outer limit of the continental shelf partly by exploitability, an unstable formula under which entitlement grew as drilling technology did.
1.3 UNCLOS III and the package deal
Two developments forced a third conference. Decolonization multiplied the number of coastal states, most of them unwilling to live under rules written by the old maritime powers. And in 1967 Malta's ambassador Arvid Pardo proposed that the deep seabed beyond national jurisdiction be declared the common heritage of mankind, administered internationally for the benefit of all, an idea the UN General Assembly endorsed in 1970.
The Third UN Conference sat from 1973 to 1982 and negotiated the text as a single package: no delegation got its preferred navigation articles without accepting the resource articles that paid for them. UNCLOS was adopted on 30 April 1982 by 130 votes to 4, with 17 abstentions, and opened for signature at Montego Bay that December. It entered into force on 16 November 1994, after a 1994 implementing agreement reworked the deep-seabed part enough to bring the industrialized states in. The package character survives in the treaty's law: Article 309 prohibits reservations, so a state takes the Convention whole or not at all.
Non-parties are not outside the system. The United States, the most consequential of them, navigates by customary international law, and much of UNCLOS now states custom. The International Court of Justice supplied the method in the North Sea Continental Shelf cases: a treaty provision may codify existing custom, crystallize a rule that was still forming, or generate new custom through subsequent state practice (ICJ, 1969). Measured that way, the 12-mile territorial sea, the basic regime of the exclusive economic zone, and the continental-shelf entitlement are all widely accepted as customary law binding everyone.
2. Maritime Zones and the Baseline
Every maritime zone is measured from a baseline. The normal baseline is the low-water line along the coast; where a coastline is deeply indented or fringed with islands, a state may instead draw straight baselines connecting appropriate points, a technique the ICJ approved for Norway's skerries in the Fisheries case and UNCLOS later codified (ICJ, 1951). The choice matters twice over: baselines fix the outer edge of every zone, and the waters landward of them are internal waters, where no foreign ship has any right of passage at all.
From the baseline outward, the Convention stacks the zones in a fixed order. Each is examined below; the map itself is compact enough to state in one table.
Zone | Outer limit | Coastal-state position | What others keep |
Internal waters | Landward of baseline | Full sovereignty | Port entry by consent; entry in distress |
Territorial sea | 12 nm | Sovereignty, incl. seabed and airspace | Innocent passage |
Contiguous zone | 24 nm | Control over customs, fiscal, immigration, sanitary matters | High-seas-type freedoms otherwise |
Exclusive economic zone | 200 nm | Sovereign rights over resources; listed jurisdiction | Navigation, overflight, cables |
Continental shelf | 200 nm, or outer edge of margin | Sovereign rights over seabed resources | Status of waters above unaffected |
High seas / the Area | Beyond national jurisdiction | None; seabed is common heritage | Six freedoms; flag-state jurisdiction |
3. The Territorial Sea and Innocent Passage
3.1 Sovereignty within twelve nautical miles
Article 2 of UNCLOS extends a coastal state's sovereignty beyond its land territory to an adjacent belt of sea, together with the seabed below and the airspace above. Article 3 finally settled the breadth question that had defeated two conferences: every state may claim a territorial sea of up to 12 nautical miles from its baselines. The 12-mile maximum is now customary as well as conventional, and the handful of wider claims that persist are routinely protested as excessive. Sovereignty over the airspace is complete; there is no aerial equivalent of innocent passage, so foreign aircraft overfly the territorial sea only by consent or under a straits regime.
3.2 What innocent passage allows and forbids
Sovereignty in this belt is crossed by the oldest servitude in the subject. Ships of all states enjoy the right of innocent passage through the territorial sea, and the analysis has two distinct limbs. First, the ship must actually be in passage: navigation that is continuous and expeditious, with stopping and anchoring allowed only as incidents of ordinary navigation, distress, or rescue. A vessel loitering without navigational purpose is not in passage and loses the right's protection whatever its intentions.
Second, the passage must be innocent, meaning not prejudicial to the peace, good order, or security of the coastal state. Article 19(2) lists twelve activities that destroy innocence, among them weapons exercises, intelligence collection, willful and serious pollution, fishing, and survey work. The dominant reading, recorded in the 1989 US–USSR Uniform Interpretation, treats the list as exhaustive: what matters is the ship's conduct in the zone, not its type, cargo, or armament. A minority of states read innocence more broadly, and a related group insists that warships need prior authorization or notification, a controversy the Convention's text deliberately left unresolved.
The coastal state is not passive. It may legislate for passage on listed subjects such as navigation safety, fisheries, and pollution, though its laws may not touch the design, construction, manning, or equipment of foreign ships except to apply generally accepted international standards. It may require tankers and nuclear-powered ships to use designated sea lanes, take necessary steps against non-innocent passage, and suspend passage temporarily in specified areas when its security requires, provided suspension is duly published and non-discriminatory.
3.3 The contiguous zone and international straits
Two adjacent regimes round out the coastal belt. In a contiguous zone extending to 24 nautical miles, the coastal state may exercise the control necessary to prevent and punish infringements of its customs, fiscal, immigration, and sanitary laws committed within its territory or territorial sea. This is enforcement competence, not sovereignty.
Straits used for international navigation get their own regime because a 12-mile territorial sea would otherwise close more than a hundred of them. The ICJ had already held in Corfu Channel that warships enjoy a non-suspendable right of passage through such straits in peacetime (ICJ, 1949). UNCLOS went further, creating transit passage: continuous and expeditious transit that cannot be suspended, that extends to overflight, and that submarines are understood to enjoy in their normal submerged mode. Transit passage was part of the price the maritime powers extracted for accepting the 12-mile territorial sea, and whether it binds non-parties as custom is still argued.
4. The Exclusive Economic Zone
4.1 Sovereign rights over resources
The exclusive economic zone was the principal invention of UNCLOS III, and it moved roughly a third of the ocean, and around ninety percent of commercial fish catches, under coastal-state resource control. The EEZ extends to 200 nautical miles from the baselines. It is neither territorial sea nor high seas but a zone of its own kind, and unlike the territorial sea it must be claimed. Nearly every coastal state has claimed one, and the ICJ recognized the zone as customary law as early as 1985 (ICJ, 1985).
Article 56 draws the zone's grammar with some care. The coastal state holds sovereign rights for exploring, exploiting, conserving, and managing the natural resources of the waters, seabed, and subsoil, together with other economic uses such as energy from water, currents, and winds, which is the textual home of offshore wind. Separately, it holds jurisdiction over three enumerated subjects: artificial islands and installations, marine scientific research, and protection of the marine environment. Rights are purposive, jurisdiction is listed, and anything falling under neither heading belongs to other states or to a residual rule of equity in Article 59.
The fisheries regime pairs conservation with access. The coastal state sets the allowable catch and must keep stocks from over-exploitation, managing toward maximum sustainable yield as qualified by environmental and economic factors; where it cannot harvest the whole allowable catch, it is to give other states access to the surplus by agreement. Enforcement powers are real but bounded: arrested vessels and crews must be promptly released on reasonable bond, and penalties for fisheries offenses may not include imprisonment absent agreement (United Nations, 1982).
4.2 Freedom of navigation in the EEZ
What other states keep in the zone is substantial. Article 58 preserves for all states the freedoms of navigation and overflight and the laying of submarine cables and pipelines, along with other internationally lawful uses of the sea related to those freedoms. Each side owes the other due regard: the coastal state must respect the freedoms, and their users must respect coastal rights and comply with lawfully adopted coastal regulations.
The case law polices the boundary. In M/V Saiga (No. 2), ITLOS held that Guinea could not apply its customs law to a tanker bunkering fishing vessels in its EEZ; the zone is not territory, and customs competence does not reach it (ITLOS, 1999). In M/V Virginia G, the tribunal accepted that regulating the bunkering of vessels fishing in the zone falls within sovereign rights over living resources, while holding that confiscating the ship went further than enforcement required (ITLOS, 2014). And the South China Sea tribunal held that claims of historic rights to resources within another state's EEZ did not survive the Convention, which allocates those resources exhaustively (PCA, 2016).
The unsettled question is military. The maritime powers treat naval exercises and intelligence collection in a foreign EEZ as internationally lawful uses related to the high-seas freedoms, subject only to due regard. A substantial minority, including China, India, Malaysia, and Brazil, require consent for foreign military activities by declaration or legislation. The Convention does not resolve the point, and the dispute now shadows most freedom-of-navigation operations in contested waters.
5. The Continental Shelf
The continental shelf is the seabed's answer to the EEZ, and the older of the two regimes. Under Article 76, every coastal state has a shelf extending to 200 nautical miles regardless of the actual geology, and, where the continental margin runs further, to the margin's outer edge within fixed constraint lines. Claims beyond 200 miles pass through the Commission on the Limits of the Continental Shelf, whose recommendations make the resulting limits final and binding.
Two features distinguish the shelf. The entitlement is inherent: it exists ipso facto and ab initio, without occupation or proclamation, as the ICJ put it in North Sea Continental Shelf (ICJ, 1969). And the rights are exclusive in a strong sense: if the coastal state does not explore its shelf, no one else may without its consent. The rights cover mineral and other non-living resources of the seabed and subsoil, plus sedentary living species. They do not affect the legal status of the waters above, so beyond 200 miles the water column over an extended shelf remains high seas. Exploitation of a shelf beyond 200 miles carries a duty to share a small percentage of revenue through the International Seabed Authority (United Nations, 1982).
6. The High Seas, Freedom of Navigation, and the Flag State
6.1 The six freedoms
Beyond every EEZ lie the high seas, roughly two-fifths of the planet's surface, open to all states, coastal and landlocked alike. Article 87 enumerates six freedoms: navigation, overflight, submarine cables and pipelines, artificial islands and installations, fishing, and scientific research. None is absolute. The last four are expressly subject to other parts of the Convention, high-seas fishing is disciplined by conservation duties and by the 1995 Fish Stocks Agreement, and every freedom is exercised with due regard for the interests of other states. Two framing rules complete the constitution of this space: no state may subject any part of the high seas to its sovereignty, and the high seas are reserved for peaceful purposes, a reservation generally read to prohibit aggression rather than naval presence as such.
6.2 Flag-state jurisdiction and its exceptions
Order on this unowned water rests on a durable fiction: every ship carries a state with it. Each state fixes the conditions for granting its nationality, a genuine link must exist between state and ship, and on the high seas a ship is subject to the exclusive jurisdiction of its flag state. The genuine-link requirement has not become a weapon against open registries; ITLOS has treated it as a duty of the flag state to exercise effective control, not a ground on which other states may refuse to recognize a ship's nationality (ITLOS, 1999). The pressure on weak flags comes instead from flag-state duties over safety, manning, and labor conditions, and from port-state inspection.
The exceptions to flag exclusivity are the entire policing architecture of the open ocean. A warship may board a foreign ship on reasonable suspicion of piracy, slave trading, unauthorized broadcasting, statelessness, or flag fraud; piracy alone attracts universal jurisdiction to seize and prosecute. Hot pursuit lets a coastal state chase a ship that violated its laws in its own zones onto the high seas, provided pursuit is immediate and unbroken. For collisions, penal jurisdiction belongs only to the flag state or the officer's state of nationality, a rule adopted precisely to reverse the Permanent Court's contrary holding in Lotus (PCIJ, 1927).
7. The Deep Seabed and Biodiversity Beyond National Jurisdiction
The seabed beyond national jurisdiction, the Area, is governed by the one part of UNCLOS that is genuinely institutional. The Area and its mineral resources are the common heritage of mankind, a principle from which no derogation is permitted, and activities there are administered by the International Seabed Authority in Kingston. The 1994 implementing agreement softened the original production controls and technology-transfer obligations enough to make the regime acceptable to industrial states, but the commercial phase has still not begun: the Authority's exploitation regulations remain under negotiation, and pressure from sponsoring states seeking to begin mining has made the mining code one of the sharpest controversies in the field (maribus, 2010).
The water column above the Area acquired new machinery recently. The BBNJ Agreement, adopted in 2023 under UNCLOS and in force since 17 January 2026, is the Convention's third implementing agreement. It covers marine genetic resources and benefit-sharing, area-based management tools including high-seas marine protected areas, environmental impact assessment, and capacity building, and it gives the two-thirds of the ocean beyond national jurisdiction its first general biodiversity regime.
8. How Disputes Are Settled
UNCLOS did something rare for a global treaty: parties consented in advance to compulsory, binding dispute settlement. Under Part XV, a state may choose its forum: the International Tribunal for the Law of the Sea, the ICJ, or arbitration under Annex VII, which serves as the default where choices do not match. The consent has limits. Disputes over a coastal state's fisheries discretion in its own EEZ are largely excluded, and states may opt out of compulsory settlement for maritime boundary delimitation, military activities, and certain law-enforcement matters. A special prompt-release procedure before ITLOS enforces the bargain that fisheries arrests end with the vessel's release on reasonable bond.
The system's reach and its limits showed in the same case. In the South China Sea arbitration, the tribunal proceeded despite China's refusal to appear, held that the nine-dash-line claim to historic rights was incompatible with the Convention, and issued an award that is legally binding and practically defied (PCA, 2016). Adjudication under Part XV keeps producing law all the same: in 2024 ITLOS delivered an advisory opinion, requested by a commission of small island states, confirming that greenhouse-gas emissions constitute marine pollution and that parties owe stringent due-diligence duties to prevent it (ITLOS, 2024).
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Conclusion
The law of the sea has held together for four decades because it is a genuine exchange rather than a code imposed by anyone. Coastal states received the EEZ and the shelf; maritime states received the 12-mile ceiling, transit passage, and secured freedom of navigation; landlocked and developing states received the common heritage principle and surplus-access rights. Article 309's ban on reservations exists to keep any state from unpicking one thread of that exchange while keeping the rest.
The open questions are correspondingly structural. Whether foreign navies may exercise in an EEZ, whether transit passage binds non-parties, whether baselines and the zones drawn from them survive sea-level rise, and whether deep-seabed mining can begin under a contested code are all disputes about the terms of the original bargain, not about its vocabulary. That the arguments are conducted inside the Convention's categories, by parties and non-parties alike, is the clearest measure of what UNCLOS achieved.
References
Agreement for the Implementation of the Provisions of the United Nations Convention on the Law of the Sea of 10 December 1982 relating to the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks (1995) adopted 4 August 1995, entered into force 11 December 2001, 2167 UNTS 3.
Agreement relating to the Implementation of Part XI of the United Nations Convention on the Law of the Sea of 10 December 1982 (1994) adopted 28 July 1994, entered into force 28 July 1996, 1836 UNTS 3.
Agreement under the United Nations Convention on the Law of the Sea on the Conservation and Sustainable Use of Marine Biological Diversity of Areas beyond National Jurisdiction (2023) adopted 19 June 2023, entered into force 17 January 2026.
International Court of Justice (1949) Corfu Channel (United Kingdom v Albania), judgment of 9 April 1949, ICJ Reports 1949, p. 4.
International Court of Justice (1951) Fisheries (United Kingdom v Norway), judgment of 18 December 1951, ICJ Reports 1951, p. 116.
International Court of Justice (1969) North Sea Continental Shelf (Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands), judgment of 20 February 1969, ICJ Reports 1969, p. 3.
International Court of Justice (1985) Continental Shelf (Libyan Arab Jamahiriya/Malta), judgment of 3 June 1985, ICJ Reports 1985, p. 13.
International Tribunal for the Law of the Sea (1999) M/V 'Saiga' (No. 2) (Saint Vincent and the Grenadines v Guinea), judgment of 1 July 1999, ITLOS Reports 1999, p. 10.
International Tribunal for the Law of the Sea (2014) M/V 'Virginia G' (Panama/Guinea-Bissau), judgment of 14 April 2014, ITLOS Reports 2014, p. 4.
International Tribunal for the Law of the Sea (2024) Request for an Advisory Opinion Submitted by the Commission of Small Island States on Climate Change and International Law, advisory opinion of 21 May 2024, ITLOS Case No. 31.
maribus (2010) World Ocean Review 1: Living with the Oceans. Hamburg: maribus gGmbH.
Permanent Court of Arbitration (2016) South China Sea Arbitration (Republic of the Philippines v People's Republic of China), award of 12 July 2016, PCA Case No. 2013-19.
Permanent Court of International Justice (1927) The Case of the S.S. 'Lotus' (France v Turkey), judgment of 7 September 1927, PCIJ Series A, No. 10.
Tanaka, Y. (2019) The International Law of the Sea. 3rd edn. Cambridge: Cambridge University Press.
United Nations (1982) United Nations Convention on the Law of the Sea, adopted 10 December 1982, entered into force 16 November 1994, 1833 UNTS 3.
United States (1945) Proclamation 2667: Policy of the United States with Respect to the Natural Resources of the Subsoil and Sea Bed of the Continental Shelf, 28 September 1945.
