Steps to reproduce
Tame, saddle, and ride a horse into water.
Observe the horse while riding it in water.
Expected result
The horse bobs up and down smoothly and moves its head smoothly.
Observed results
The horse visually stutters once every 1 or 2 seconds.
This seems to be a result of the fix for (MCPE-231719). It looks like a slight desychronization between the horse's server-authoritative bobbing in water and its client-authoritative player-controlled movement.