Regardless of how they react to the water, it would still be a better deterrent than none. In Walking dead the zombie caught only partially submerged in the well fell apart when being pulled up. So if they decompose to some extent that makes it hard to zombie as boost said. And even if they were just fine, they wouldn't be able to climb the side of a large boat, or likely to make it to a distant island in large enough groups to cause damage. Not to mention, if I lived on an island, then they would have to deal with the large ditch I dug around the outer portion of the island.

But instead, let's just hole up inside of a zombie infested prison, or play house on a farm.