What will happen to the legal status of sinking nations when their land is gone?

countries own the water along their costs usually for a good bunch of miles, so why wouldn't a country be able to exist and only own water?

that being said i guess the flipside is that if certain countries now _want_ another place not to be a state, they just decide it be so regardless of this place having land or not.... so maybe it's a moot point and the state is only allowed to be there if other's with big guns allow it to be so...