When I was playing the new snapshot I was testing out horses and rode mine into a lake about 7 blocks deep. As I was dismounted because of the water my horse sunk and grazed at the bottom of the lake until it drowned.

What I expected to happen was for the horse to float so I could use the lead to lead it out of the lake

The horse sunk and grazed on the bottom until it drowned

1. ride horse into water.
2. Horse will sometimes sink
3. Horse will graze on the bottom until it drowns