I followed 2 zombies that were running away from me towards something near the coast that I could not yet see. Once they reached the water they started walking slowly out to sea. I swam after them to see what was going on, and I could see under the water that there were about 10 other zombies already about 50 meters out walking after a cow that was also moving out to sea.

Given that I see zombies chase animals all the time (which is not necessarily a bad thing!), the animal AI needs fixing so that they don't run in the water, and zombies should die underwater.