[Endoperez]

I'm not sure if the existing definition makes difference between land-coastal and sea-coastal provinces, but at least land provinces neighbouring seas have some special features (Shambler Reef and other sites, "some water gems have washed ashore" and other events, certain poptypes, etc).
It might be that some underwater poptypes (Mermen) only appear on sea provinces neighbouring land provinces. I don't know what and where this affects. The Mermen are the only one I can think of right now. However, there can very well be sites/events that are limited to "shallow", i.e. land-neighbouring, seas.