Okay, so mermaids and sirens are salt water. Sea creatures. Nymphs, rhinemaidens - kind of like the equivalent for fresh water. You don't get nymphs in the sea and you don't get mermaids in a lake.

( a pause. )

I don't know how similar, exactly, there are probably more differences than just where we're made for, but I know a lot of the same things hold true.