From a physical perspective, most zombies, as typically depicted, are too decayed to float, and the bits where gas could build up tend to be open. Moving around would also tend to burst anything like that.
So I gotta go with sink. The idea of swimming zombies also seems inherently dumb to me, but I don't generally like intelligent/competent zombies unless they're "zombie lord"-types.
Plus it's way scarier when something bursts out of the water rather than sort of bobbing along slowly and ridiculously (presumably doggy-paddling at most, no-one way any zombie can do an actual swimming stroke) for several minutes before it reaches you.