[Edi]

In CoE3 there are no provinces, there are squares. In water squares, there is nothing. No underwater forts, no resources or anything else so far. So water can be safely ignored unless somebody uses amphibian units to sneak attack from the sea. Not a great concern most of the time.

Hiding out in the ocean is also not going to do anyone any good, since as soon as they lose all forts, they are eliminated.