The Economist explains

How are maritime boundaries determined?

The UN convention is widely accepted, even where not formally agreed

A CANNON SHOT fired from the coast can hit a ship three nautical miles out to sea. In the 17th century, this was the basis for maritime boundaries. Countries could claim sovereignty as far as they could defend from the shore. Today land is still the basis of claims over the sea, and with countries projecting their military and economic power farther across the waves, competing claims have left the oceans fraught with disputes: 39% of sea boundaries are yet to be agreed. However, this matters less than may be expected.

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