Usually, the baseline from which the coastal sea is measured is the low-water line along the coast, as indicated on large-format maps officially recognized by the coastal State. This is either the closest low-water mark to shore or an unlimited distance of permanently exposed land, provided that some of the altitudes exposed to low tide but covered at high tide (such as mudflats) are within 3 nautical miles (5.6 kilometers; 3 + 1 ⁄ 2 legal miles) of permanently exposed land. One can also define straight baselines that connect the islands bordered along a coast, through the mouths of rivers or, with certain restrictions, through the mouths of bays. In this case, a bay is defined as « a well-marked indentation whose penetration is so proportional to the width of its mouth that it contains inland waters and represents more than a simple curvature of the coast. However, an indentation is considered a bay only if its surface is as large or larger than that of the semicircle, the diameter of which is a line drawn above the mouth of that indentation. The baseline across the bay also cannot exceed 24 nautical miles (44 kilometers; 28 legal miles) long. The U.S. Exclusive Economic Zone (EEZ) extends 200 nautical miles from the baseline of the coastal sea and borders the U.S. Territorial Sea by 12 nm, which straddles the contiguous zone by 12 to 24 nm. Within the EEZ, the United States has: NOAA is responsible for representing on its nautical charts the boundaries of the territorial sea of 12 nautical miles, the adjacent area of 24 nautical miles and the exclusive economic zone (EEZ) of 200 nautical miles. These areas are highlighted in orange. Areas on the adjacent high seas differ from territorial waters proper, where coastal States do not claim territorial rights but claim jurisdiction limited to one or more special purposes. These contiguous areas of 6 to 12 nautical miles (11 to 22 km) outside territorial waters are most often used for customs and hygiene enforcement, but in some cases they may be established for fisheries protection or security. Territorial waters also differ from claims made by many states after 1945 to the continental shelf off their coasts, where potentially valuable resources could be present.

Such claims met with few objections from other States if they were limited to the plateau itself without affecting the state of the waters above the high seas, but the measures taken by some States, such as Chile, Ecuador and Peru, asserting their jurisdiction over the waters and shelf up to 200 nautical miles (370 km) offshore, provoked numerous protests, as they amounted to unacceptable extensions of territorial waters. The territorial sea is defined under UNCLOS as the area of 12 nautical miles from the baseline or low-water line along the coast. The birth of a nation`s authority to control its coastal waters dates back to the sixteenth century. In the seventeenth century, the father of international law and the doctrine of freedom of the seas, Grotius, recognized the existence of national jurisdiction over coastal waters that could be effectively controlled from land. The extent to which a nation could control its coastal waters depended largely on the range of its guns on the coast. Thus, the limit of 3 nautical miles (nm) of the territorial sea results from what is often called the « cannon fire » rule. In the United States, the creation of a territorial sea and contiguous zone dates back to the late 1700s in response to national security and law enforcement problems in coastal areas, including a 1793 diplomatic note by Thomas Jefferson and a law passed by Congress in 1799 that allowed the boarding of foreign-flagged ships within 12 nautical miles of the coast. This area was known as « customs waters » and was later called a « contiguous zone ».

In 1930, the coastal State`s property rights over the resources of the territorial sea were well established, provided that they did not interfere with a ship`s right to peaceful passage. In 1945, President Truman issued a proclamation affirming the right to explore and exploit the oil and gas resources of the continental shelf outside the territorial sea of the 3 seas. In 1953, the U.S. Congress enacted laws on federal and state control of the continental shelf. In 1958, international conventions on the territorial sea, the contiguous zone and the continental shelf were concluded in Geneva. The concept of a fisheries protection zone was born in another truman proclamation, but Congress did not pass a 200 nm fisheries protection zone law until 1976. This developed into an area where a coastal nation had exclusive control over all economic exploration and exploitation of natural resources off its coasts. Until 1982, the custom of asserting a 12 nm coastal sea, a 24 nm contiguous zone and a 200 nm EEZ was codified in the United Nations Convention on the Law of the Sea (UNCLOS).

The maritime boundaries and borders of the United States are measured by the official U.S. baseline, which is recognized as a low-water line along the coast, as indicated on noaa charts under the articles of the Law of the Sea. The Office of Coast Survey shows on its nautical charts the coastal sea (12 nautical miles), the adjacent area (24 nm) and the exclusive economic zone (200 nm, plus maritime boundaries with neighboring/opposite countries). Another challenge concerns the definition of islands. States are encouraged to obtain island status because of their characteristics on the high seas. Unlike rocks or low water levels, islands project a full coastal sea with overflight control and a full EEZ. This problem is most prevalent in the South China Sea, which is resource-rich and contains many maritime features that may or may not be islands eligible for a large EEZ. Even small islands, such as the Spratly Islands, which total 1.5 square miles, can project hundreds of square nautical miles of exclusive economic control over the South China Sea. This issue is discussed in more detail in Chapter Ten: The South China Sea Tribunal. States may also establish a contiguous zone from the outer edge of coastal seas up to a maximum of 24 nautical miles from the baseline. This zone exists to strengthen a state`s law enforcement capacity and prevent criminals from fleeing the territorial sea.

In the contiguous zone, a State has the right to prevent and punish violations of tax, immigration, health and customs laws on its territory and in the territorial sea. Unlike the territorial sea, the contiguous zone confers jurisdiction over the surface and bottom of the ocean on a single State.3 It does not grant air and space rights. Administrative borders are the territorial maritime boundaries within which French and British sovereignty prevails, including those specific to the Channel Islands (atlas-transmanche.certic.unicaen.fr/en/). Maritime responsibility defines where french and British authorities are allowed to intervene in an event such as a shipwreck, and there are rescue and surveillance centres at sea in France (CROSS: Regional Operational Centres of Surveillance and Sea Rescue) and England (MRCC: Maritime Rescue Coordination Centre). The fishing zones established by the International Council for the Exploration of the Sea (ICES) cover the entire English Channel (Fig. 6.1.). Unlike other areas whose existence stemmed from ancient international law, the EEZ was a creation of the LOSC. States can claim an EEZ that extends 200 nautical miles from the baseline. In this area, a coastal State has the exclusive right to exploit or conserve all resources in the water, on the seabed or under the subsoil of the seabed. These resources include both living resources such as fish and non-living resources such as oil and natural gas.4 States also have the exclusive right to produce offshore energy from waves, currents and winds in their EEZs. Article 56 also allows States to establish and operate artificial islands, facilities and structures, conduct marine scientific research and protect and maintain the marine environment through marine protected areas.5 Article 58 provides that Articles 88 to 115 of the Convention on the Rights of the High Seas shall apply to the EEZ « to the extent that they are not inconsistent with this Part [V] ». 6 Conflicts have arisen whenever a coastal nation claims an entire Gulf as its territorial waters, while other nations recognize only the more restrictive definitions of the United Nations Convention.

Claims that establish a baseline of more than 24 nautical miles (two limits of 12 NM) are considered exaggerated by the United States. Two conflicts have occurred in the Gulf of Sidra, where Libya has drawn a line of more than 230 nmi (430 km; 260 mi) and claimed the entire closed gulf as its territorial waters.