International waters

The terms international waters or transboundary waters apply where any of the following types of bodies of water (or their drainage basins) transcend international boundaries: oceans, large marine ecosystems, enclosed or semi-enclosed regional seas and estuaries, rivers, lakes, groundwater systems (aquifers), and wetlands.^[1]

"International waters" is not a defined term in international law. It is an informal term, which sometimes refers to waters beyond the "territorial sea" of any country.^[2] In other words, "international waters" is sometimes used as an informal synonym for the more formal term "high seas", which under the doctrine of mare liberum (Latin for "freedom of the seas"), do not belong to any state's jurisdiction. As such, states have the right to fishing, navigation, overflight, laying cables and pipelines, as well as scientific research.

The Convention on the High Seas, signed in 1958, which has 63 signatories, defined "high seas" to mean "all parts of the sea that are not included in the territorial sea or in the internal waters of a State" and where "no State may validly purport to subject any part of them to its sovereignty."^[3] The Convention on the High Seas was used as a foundation for the United Nations Convention on the Law of the Sea (UNCLOS), signed in 1982, which recognized exclusive economic zones extending 200 nautical miles (230 mi; 370 km) from the baseline, where coastal states have sovereign rights to the water column and sea floor as well as the natural resources found there.^[4]

The high seas make up 50% of the surface area of the planet and cover over two-thirds of the ocean.^[5]

Ships sailing the high seas are generally under the jurisdiction of the flag state (if there is one);^[6] however, when a ship is involved in certain criminal acts, such as piracy,^[7] any nation can exercise jurisdiction under the doctrine of universal jurisdiction. International waters can be contrasted with internal waters, territorial waters and exclusive economic zones.

UNCLOS also contains, in its part XII, special provisions for the protection of the marine environment, which, in certain cases, allow port States to exercise extraterritorial jurisdiction over foreign ships on the high seas if they violate international environmental rules (adopted by the IMO), such as the MARPOL Convention.^[8]

Underground Transboundary Waters

When an underground water crosses international boundaries, the term transboundary aquifer applies. The term transboundariness can then be applied, which is a concept, measure and approach first introduced in 2017 ^[9] when talking about underground transboundary waters.

The importance of this approach is that the physical properties of aquifers become merely additional variables within the broad spectrum of the transboundary nature of an aquifer: social (population); economic (groundwater efficiency); political (cross-border); existing research or data; water quality and quantity; other issues that drive the agenda (security, trade, immigration, etc.).

The criteria proposed through this approach attempt to encompass and quantify all potential variables that play a role in defining the transboundary nature and multidimensional boundaries of an underground transboundary water.

International waterways

The Río de la Plata basin gives sea access to landlocked Paraguay and Bolivia, and navigation is free for all international commercial ships.

Komárno in Slovakia is an inland port on the Danube River which is an important international waterway.

Several international treaties have established freedom of navigation on semi-enclosed seas.

The Copenhagen Convention of 1857 opened access to the Baltic Sea by abolishing the Sound Dues and making the Danish straits an international waterway free to all commercial shipping. Separately, the Royal Ordinance of 1999 regulates the access of foreign warships to Danish waters.^[10]^[11]^[12]
Several conventions have opened the Bosphorus and Dardanelles to shipping. The latest, the Montreux Convention Regarding the Regime of the Turkish Straits, maintains the straits' status as an international waterway.

Other international treaties have opened up rivers, which are not traditionally international waterways.

The Río de la Plata basin, including the rivers Paraná, Uruguay and Paraguay, is legally open for all international commercial ships without restriction, it notably gives sea access to landlocked Paraguay and Bolivia.
The Danube River is an international waterway so that Germany and Croatia, as well as landlocked Austria, Slovakia, Hungary, Serbia and Moldova can have secure access to the Black Sea.

Disputes over international waters

Current unresolved disputes over whether particular waters are "International waters" include:

Arctic Ocean: While Canada, Denmark, Russia and Norway all regard parts of the Arctic seas as national waters or internal waters, most European Union countries and the United States officially regard the whole region as international waters. The Northwest Passage through the Arctic Archipelago is one of the more prominent examples, with Canada claiming it as internal waters, while the United States and the European Union considers it an international strait.^[13]
Southern Ocean: Australian claim to an exclusive economic zone (EEZ) in Antarctica is challenged by Japan.
Okinotorishima: Japanese claim to an EEZ around Okinotorishima is challenged by China, South Korea, and Taiwan.
South China Sea: Disputes exist between Brunei, China, Indonesia, Malaysia, the Philippines, Taiwan, and Vietnam.