The first change is who controls the water
A ship crossing the edge of a country's territorial sea leaves waters where that country exercises sovereignty much like it does on land. Under international law, a territorial sea can extend up to 12 nautical miles from a coastal baseline. The crossing does not necessarily put the vessel in the open ocean, though: it may enter the country's exclusive economic zone, which can reach 200 nautical miles from shore.
An exclusive economic zone is not national territory. The coastal country has special rights there to explore and manage resources such as fish, oil, and gas, while ships of other countries generally retain the right to navigate through it.
The ship still carries its country's rules
Farther out, on the high seas, no coastal country has general sovereignty over the water. But a ship does not become legally unclaimed. It is ordinarily governed by the laws of the country whose flag it flies, and international agreements set shared rules for matters such as safety and pollution.
That means a captain's obligations do not vanish at a maritime boundary. The flag state's authority travels with the vessel, while other countries may still have specific powers under international law—for example, to act against piracy.
A boundary changes the balance, not the need for rules
For a sailor, the practical shift is from one coastal state's broad territorial authority to a more layered system: resource rights may belong to a nearby country, navigation remains widely protected, and the vessel's flag state remains central. The ocean can be beyond any country's ownership without being beyond law.
