Some mobs like sheep or horses do not drown in water if they are trapped floating up against blocks (pigs and foxes, for example, do drown).

Steps to reproduce:

1. Find an ice river/ice ocean or a water-filled cave (or place blocks in water).
2. Summon a sheep or a horse under the ice/cave ceiling/blocks (more mobs may work, requires further testing).
3. The mob now floats up against the ceiling.
4. It does not drown despite being trapped in water.

Edit: After further testing I found that the bug also affects cows, donkeys, mules, llamas, camels, chickens, ocelots and probably many more types of mobs.