I don't think Walking Dead has specifically dealt with the issue of water. In World War Z, zombies who ended up in the water somehow would just float on the currents until they reached land and then continue their zombie existence. So an island would be safer than the mainland, but there would always be the danger of a random zombie washing up.
I think it would be pretty far odds that a zombie could ever make it to an island via currents.
1. I think the salt water would corrode their dead flesh before they got that far.
2. Any type of storm combined with the salt water and sun would batter or bloat a dead body apart.
At some point they would probably bloat to the point of popping and then just sink.
3. Sharks and fish would eat or peck at them until there was nothing left of them.
4. Lastly I think the odds of making it from point "A" to "B' would be pretty astronomical.