Wrongo my friend. Zombies have no sense of water pressure or anything, everything is senseless and they will keep going.
Find some solar panels, get some friends who know what they are doing, and go to Costco.
I didn't mean the feeling of water pressure(zombies can't feel anything), I mean they might be crushed cause I'd be out in the middle of the ocean. Not just some off-shore thing.
Besides, it doesn't matter if they keep going, they still won't make it. In a submarine, go up. In a boat, don't use an anchor. On an island, the rocks are hard to climb.(For a small amount of time, eventually some zombies will make it)
Although, considering that their brains could be the last thing that gets squeezed, pressure may not be effective at all. Perhaps one would need to find a deep point in the ocean and float around there in a boat/submarine.
[Edit] I'm reading the survival guide, started before this thread appeared.
[Edit2] Both plans require the same materials, though, and getting on a building is more likely than getting on a boat.
[Edit3]
someone has obviously not read "Max Brooks Zombie Survival Guide"
good catch Bokk
Look carefully in my post, I specifically referenced the Zombie Survival Guide.