As near as I've been able to suss it out, the wave action or currents washes of the decaying flesh and they disintegrate. No personal experience though.
That or schools of fish come along for a nibble and just don't stop. Oh the irony!
Mental note to self - Don't eat fish from sources known to have had zombies walk across.
About the too slow comment from Tengu. Obviously the zombies I'd need to get away from are the Resident Evil 1 variety, who shuffle about aimlessly looking for a snack. The Resident Evil 3 and World War Z zombies will be banned from my own personal Zombie Apocalypse. Heck, in that world even Roller skates would be an asset..... Hmmmmm 4x4 rough terrain roller skates. Now
there's a gap in the market that could be exploited. This time next year Rodders (or Zombie Apocalypse, whichever comes first) I'll be a mullyanaire!
But then there's always this: