It's called water. It's a magical shield where zombies go in the daytime to have fun and to make a splash, and where skeletons go to wash their dusty bones. And it's driving the Steves insane.
Imagine a beautiful beachfront manor. Now imagine sleeping in that beachfront manor. Now imagine waking up and walking onto the cool morning sand. Now imagine looking at the ocean, realizing that it is filled to the brim with decayed or decaying creatures of the night, floating around like the sun is nothing. Swimming. Waiting. Waiting for the night to come so they can crash your nightly
Now, this doesn't make sense. The sun should burn them. Sure, the water may prevent combustion, but the heat is still on them. Even if they aren't on fire, they should still burn. Don't you sunburn in the water? Yes. You do. Why should the undead be any different?
It's also not very fun, gameplay-wise. Who would want a swarm of angry undead enemies swimming around when they SHOULD be dead, when they SHOULD be gone, during broad daylight?
Which is why I suggest that the Undead be damaged by the sunlight when in the water, even if they don't combust. Their AI should also encourage them to go into the shade. Naturally, this wouldn't be very nice to the Undead, which is why I also suggest that Undead coming out of the water recently will be damaged, but not combusted by the fire, allowing them time to escape into a nice dark cave without forcing them to set themselves on fire. Not only is this a better alternative to have them jump into a pool of water for an entire day, it makes more sense and is just plain cooler.
Mr. Gorbachev. Tear down this aquatic wall!