When a rider entity is controlling a mount, and the mount is in deep water, they will both sink, as the controlling entity doesn't know to float in the water while riding another entity.

Steps to Reproduce:

1. Enter a Creative world.
2. Above or near a deep body of water, spawn a pig and some baby zombies. Eventually a baby zombie jockey will spawn.
3. Switch to survival mode and approach the baby zombies. If there is a baby zombie jockey, it will quickly mount the pig.
4. Switch back to creative mode and push/drop the baby zombie riding the pig into the water.
5. Notice how it sinks instead of floating like either the pig or the baby zombie would do if they were separate.