|boundary = maritime|
Almost every non-landlocked country has maritime borders, national borders at sea. United Nations Convention on the Law of the Sea establishes that the territorial waters of a country can extend up to 12 nautical miles (22.227 kilometres) from the low-tide mark of the sea (unless such extension conflicts with territorial waters of another country). This distance does not apply for all borders of this type: for instance, the English Channel is only 34 km at it's narrowest point. In this situation, the national border lies at the midway point of the Channel. However, a significant number of country borders lie 22 km from their coastline.
When drawing coastlines in OpenStreetMap, the rule is “water on the right, land on the left” (coastlines of islands will always be drawn counterclockwise), thus a maritime border will always be to the right of the coastline. However these maritime borders don't have a restriction on their direction of drawing, they are just unoriented polygons, theoretically delimiting areas on the sea.
Maritime borders are frequently not fully closed, notably for the exclusive economic zone or the extension of the continental shelf (due to the absence of international agreement between two or more bordering countries). If some part of the EEZ is missing, the area is implicitly limited by the territorial waters and by the shortest segments joining each ending node of the EEZ to the territorial waters; when territorial waters are not delimited between two countries, they are also limited by their contiguous zone or if it can be defined, otherwise by their baselines; in frequent cases, there's not even any baseline clearly defined and these maritime areas will be limited to the coastline.
The territorial waters line (12 nm zone) should be included in boundary relations, the same way as land borders.
If a country has defined the maritime borders of sub-national entities, this way should be included in boundary relations for these entities.
In the United States of America, NOAA nautical charts specify in Note X:
"Within the 12-nautical mile territorial sea, established by Presidential Proclamation, some federal laws apply. The Three Nautical Mile Line, previously identified as the outer limit of the territorial sea, is retained as it continues to depict the jurisdictional limit of the other laws. The 9-nautical mile Natural Resource Boundary off the Gulf coast of Florida, Texas, and Puerto Rico, and the Three Nautical Mile Line elsewhere remain in most cases the inner limit of Federal fisheries jurisdiction and the outer limit of the jurisdiction of the states. The 24-nautical mile Contiguous Zone and the 200-nautical mile Exclusive Economic Zone were established by Presidential Proclamation. Unless fixed by treaty or the U.S. Supreme Court, these maritime limits are subject to modification."
Therefore, OpenStreetMap should extend state and county boundaries 3 nautical miles offshore (or 9 nautical miles offshore where applicable). These boundaries should not hug or intersect or roam around the coastline except where such boundaries separate one county (or county equivalent) from another.
The baseline from which the territorial sea is measured is mainly the low-water line along the coast as marked on large-scale charts officially recognized by the coastal state. Unfortunately, natural=coastline should be placed at mean high water spring (MHWS). The base line draws often lines across fjords or gulfs, all navigational waters inside the base line are defined as inshore or protected water. It also exist straight baselines or archipelagic baselines.
A state's territorial sea extends up to 12 nautical miles (22 km) from its baseline. If this would overlap with another state's territorial sea, the border is taken as the median point between the states' baselines, unless the states in question agree otherwise. A state can also choose to claim a smaller territorial sea (usually 3 nm). Some states do claim territorial sea up to 200 nautical miles (this does not meet the requirements of UNCLOS). There are also some other special cases like the territorial sea of the Philippines based on coordinates. The UN provides a list with maritime claims.
- boundary=administrative since so many had a problem with grouping it together with maritime borders
- maritime=yes to state that this is a maritime border, so it can be rendered correctly/different from land borders
- admin_level=2 since it is a national boundary
- border_type=territorial to distinguish it from borders on land
This line should also be member of the border relation surrounding the country.
This line differs from the other maritime boundaries as a result of the vote on Maritime Borders. The agreement was to keep it as boundary=administrative as the land border, but tag it with maritime=yes.
- Potential data sources
- US 
The contiguous zone is a band of water extending from the outer edge of the territorial sea to up to 24 nautical miles (44 km) from the baseline, within which a state can exert limited control.
- Potential data sources
- US 
An exclusive economic zone extends from the outer limit of the territorial sea to a maximum of 200 nautical miles (370.4 km) from the territorial sea baseline, thus it includes the contiguous zone. A coastal nation has control of all economic resources within its exclusive economic zone, including fishing, mining, oil exploration, and any pollution of those resources.
This is a special border to mark the boundary between IALA Region A and IALA Region B, needed to distinguish the form of Lateral Sea Marks used. This boundary should not be edited, unless the board of IALA wishes to change this system. Region B is North America, South America, Caribbean, Japan, Republic of (South) Korea, the Philipines. Region A is the rest of the world.
The border of admin_level=2 is not where two entities of admin_level 2 meets, but a continuous line around one entity of admin_level 2, therefore use admin_level 2 on the territorial border.
- United Nations Laws of the Sea: The UN DOALOS have a database over all maritime border claims. There is no apparent license on the data and this is the borders definitions. But the database is a database of different PDF files with coordinates of baselines and territorial borders. The text has to be parsed by hand and other maritime borders needs to be calculated from the baseline (see maritime borders). Data is to be tagged with source=UNCLOS