We have seen what happens to zombies when they're stuck in water for prolonged periods of time. There was one that had been trapped in a well on Herschell's farm for quite some time. He was pretty gross, but still a threat once they got him out.
It doesn't really matter though. Regardless of where the characters go -- an island, an underground bunker, a space station on the moon -- they won't be safe. If they were, there wouldn't be any point to the show.