How Would I Create a Swimming Mechanism?

I have tried numerous different things to get swimming to work for one of my games; however, none of them are quite what I'm aiming for. Is there a way to make any player that falls beneath a certain height (say, Y=0, for instance), float at that height, yet still be able to move and jump onto parts higher than that height? It sounds fairly simple, but I'm not very familiar with BodyPositions or anything like that. Any help would be much appreciated. Thanks in advance.