How do countries measure their ocean borders?

Question

How do countries measure their ocean borders?

Answer 1

5 miles from any point on the shore.

there r special cases

for example here in canada we own the ocean between our islands in the north.

americans russians and the british hate us for that, cause we own the potential northwest passage that would open up year round if global warming continues.

so there are special cases, but these are often subject to debate, criticism and speculation

Answer 2

Um, first answer was incorrect in that under international law, countries own 10 miles from their shore. some try to claim more, i.e. China...

The stuff about Canada is true, though.