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