The bug
Zombie horses can't be ridden underwater even though they sink like skeleton horses.
How to reproduce
/summon zombie_horse ~ ~ ~ {Tame:1b}
Code analysis by @unknown can be found in this comment.
Linked issues
Attachments
Comments
Confirmed in 20w16a
Confirmed in 20w22a. I'd also like to clarify that this affects all entities, not just players.
I'm not sure just how different the Forge library is from Minecraft's actual code, but looking at Forge public boolean canBeRiddenInWater() {
return true;
}
can be found in the SkeletonHorseEntity class but not the ZombieHorseEntity class. Simple fix?
Can confirm in 21w03a.
Can confirm in 21w05a.
Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 21w07a.

This is a feature request.
Confirmed in 1.20
Confirmed in 1.13.1