A maritime border is the boundary that defines how far a country’s legal authority extends into the ocean. Unlike borders on land, which are typically a single line, maritime borders are drawn as a series of expanding zones radiating outward from a country’s coastline. Each zone grants the coastal state different levels of control, from full sovereignty near shore to limited economic rights farther out. The framework governing these zones is the United Nations Convention on the Law of the Sea (UNCLOS), agreed upon in 1982 and now accepted by more than 160 countries.
How Maritime Zones Work
Maritime borders aren’t one line in the water. They’re layered. Picture a series of concentric rings extending from a country’s coast, each one granting fewer rights than the last. The distances are measured in nautical miles (one nautical mile equals about 1.15 standard miles or 1.85 kilometers), and each zone has its own set of rules about what the coastal state can and cannot do.
The baseline from which all these zones are measured is typically the low-water line along the coast. Everything is counted outward from there.
The Territorial Sea: 0 to 12 Nautical Miles
The territorial sea extends 12 nautical miles from the baseline, and it’s the zone where a coastal state has the most power. The country exercises full sovereignty over this water, the airspace above it, and the seabed below it. For practical purposes, the territorial sea is treated almost like land territory. The state controls fishing, resource extraction, and law enforcement within it.
There is one important exception: foreign ships have the right of “innocent passage,” meaning they can travel through territorial waters as long as they do so continuously and without threatening the coastal state’s security. A cargo ship transiting a strait, for example, doesn’t need permission. A warship conducting weapons exercises would be a different story.
The Contiguous Zone: 12 to 24 Nautical Miles
Beyond the territorial sea lies the contiguous zone, stretching from 12 to 24 nautical miles offshore. Here, a country no longer has full sovereignty, but it retains the right to enforce four specific categories of law: customs, taxation, immigration, and sanitation. Think of it as a buffer zone. If a ship is approaching with contraband or undocumented passengers, the coastal state can intercept it in the contiguous zone rather than waiting until it crosses into territorial waters.
The Exclusive Economic Zone: Up to 200 Miles
The exclusive economic zone, or EEZ, is where maritime borders start to matter enormously for a country’s wealth. It extends up to 200 nautical miles from the baseline, and within it, the coastal state has sovereign rights over all natural resources. That includes fish, oil, natural gas, minerals on the seabed, and even energy generated from water currents and wind. The coastal state decides how much fish can be caught in its EEZ and sets the rules for who gets to catch it. If a country can’t harvest its entire allowable catch, it’s expected to grant other nations access to the surplus.
The EEZ is not sovereign territory, though. Foreign ships can navigate freely, aircraft can fly over it, and other countries can lay submarine cables and pipelines through it. The coastal state controls the resources, not the movement of people and vessels.
The Continental Shelf
The continental shelf refers to the underwater extension of a country’s landmass, the submerged seabed and subsoil stretching out from the coast. Under UNCLOS, a coastal state has rights to the resources on and beneath its continental shelf out to at least 200 nautical miles. But if the physical continental margin extends farther than that, the state can claim rights beyond 200 miles by submitting scientific evidence to a UN body called the Commission on the Limits of the Continental Shelf.
This process is active and ongoing. As of late 2025, 98 submissions have been filed by countries seeking to establish extended continental shelf claims, with recent filings from Fiji, the Solomon Islands, Vanuatu, and Chile. Russia has submitted a revised claim in the Arctic Ocean near the Gakkel Ridge, reflecting how competition for seabed resources in remote areas continues to intensify. These claims matter because the continental shelf can contain vast oil, gas, and mineral deposits.
The High Seas: Beyond National Jurisdiction
Beyond the EEZ and continental shelf lie the high seas, which belong to no country. All states, whether they have a coastline or not, enjoy a set of freedoms here: navigation, overflight, fishing, laying submarine cables, building certain installations, and conducting scientific research. No nation can claim sovereignty over any part of the high seas.
This doesn’t mean the high seas are lawless. International treaties govern activities like fishing, pollution, and piracy. But enforcement is far more difficult when no single country has jurisdiction.
Why Maritime Borders Matter
Maritime borders carry enormous economic and security stakes. Billions of dollars’ worth of trade passes through critical waterways like the South China Sea, the Strait of Malacca, and the Indian Ocean every year. Protecting shipping lanes, preventing illegal fishing, and stopping drug trafficking all depend on clearly defined maritime boundaries and the ability to enforce them.
Illegal, unreported, and unregulated fishing is a growing concern. It threatens both the economic livelihood and food security of coastal nations, particularly in the developing world. The United States alone has provided $136 million since 2017 to maritime law enforcement partners in the Indo-Pacific to help them protect their sovereign waters and interdict illicit activity.
Contested maritime claims also create geopolitical flashpoints. China’s expansive claims in the South China Sea, which overlap with the EEZs of several neighboring countries, have led to military buildup on artificial islands and repeated confrontations with other nations’ vessels. Disputes like these illustrate how maritime borders are not abstract legal concepts. They shape international relations, military strategy, and the global economy.
How Maritime Border Disputes Get Resolved
When two countries’ maritime zones overlap, or when one country challenges another’s claims, UNCLOS provides mechanisms for resolution. Countries that have ratified the convention can choose from several options: the International Tribunal for the Law of the Sea (ITLOS), based in Hamburg, Germany; the International Court of Justice; or binding arbitration. ITLOS specializes in disputes over territorial sea boundaries, EEZ limits, and continental shelf claims.
Disputes over the delimitation of overlapping zones are among the most common cases. When two countries sit less than 400 nautical miles apart, their 200-mile EEZs would theoretically overlap. In these situations, the boundary is typically negotiated or, failing agreement, decided by an international court. The principle of equidistance, drawing a line equidistant from both coastlines, is a common starting point, but geography, the presence of islands, and historical claims all complicate matters.
Not all countries accept these mechanisms, however. Some major maritime powers have opted out of compulsory dispute resolution for certain categories of claims, which can leave contested boundaries unresolved for decades.