Chain-mail and boats don't go together well, but maybe kevlar could do the same job (if you can access it in time).
as for Zombie's walking underwater, there is little evidence to support this. Rotting flesh generates methane gasses that would make the undead float to the top, surely? Anyways, they would have to be led to the island fortress, I doubt they would just wander to it without any kind of bait. That said, the island would need to be fortified against just such an attack, and include a quarantine area for any new arrivals.
Crossbows are awesome if your dealing with small groups or lone walkers. What is needed is a weapon of mass destruction for if we're overrun. The Toyota Tundra was the best I could come up with. Obviously there are better weapons, but can I really access them once the virus hits? Prolly not.