Mobs riding mobs (spider jockey, chicken jockey, etc.) will dismount underwater. It's difficult to keep them riding each other in wet places.
To reproduce:
1. Spawn a riding mob
2. Throw it into deep water
3. Mobs will dismount
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Command used?

May be intended since the player too dismounts ridden mobs when going into water.

So, it should work only if player is riding.

If it "should" work only with players, then this is not a bug, but a feature request.

It shouldn't work with players! We have to be able to swim on horse.

This should be only for player-horse, as it hinders the development of custom maps.

Confirmed for 1.8.4. Also this should be renamed to Mobs riding mobs dismount in water deeper than 2 blocks, since like players riding horses, the mobs only dismount if the water if deeper than 2 blocks.

Confirmed for
15w39c

Gave the ticket to Marcono1234.
Confirmed for 15w41b

Confirmed for 15w45a

Confirmed for 15w46a

Confirmed for 15w47b