That's true except for when there is something in the water that a powerful nation wants to exploit, then the line can be pushed from the land itself to the edge of the continental shelf and if that's not far enough for those purposes then just make up a reason.International waters begins at 200 nautical miles from land, or 230.15 miles / 370.4km.
So what happens to places like the Gulf of Mexico? Well, it actually has two donut holes worth of international waters, on the maritime boundaries of the USA, Mexico, and on the eastern one, Cuba.
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There are two donut holes that are entirely surrounded by a single nation:
Russia has one
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and Japan has one
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The green area is currently disputed by Iceland, Denmark and Ireland.
The reason China is building up those rocks and atolls and stationing (military) people on them to extend their border is because under UNCLOS "land" must be able to sustain human habitation. China is blatantly exploiting the rules of the UN.