People living on cruise liners which have been retrofitted with desalinization equipment, big floating cities reminiscent of Water World, the difference being there is land and we know where it is but we cant go there, because it’s infested with Zombies.
Not only do you have to worry about resource gathering, and general survival on the seven seas but you also have to deal with the odd floater (floating Zombies?) that passes by.
People will want to get on land to harvest raw materials and food stuff, but they run the danger of being eaten alive by the undead. I also envision there being floating zombie ships where all the occupants have been turned or eaten because the ship ran aground or they took on someone who’d been bitten.
It’s an Idea for thought, I’d be surprised if there isn’t already a B-Movie that has this very premise.