Poseidon and the Waters

Poseidon is the highest God of the Sea, and also the God of Rivers, which create creeks and streams as well. So would it be correct to call Poseidon the God of Water in general?

This is correct. Poseidon is the High King (best term I can think of), and all the sea nymphs, fresh water nymphs (nymphs of springs, etc) and river gods are rulers in their own right, but their rulerships are the water source over which they guard and preside. Poseidon is the High King under Whose auspices they rule under.