Hi,
When leading a horse across a small river etc they still feed like on grass however this stops it from swimming and sinks.
If he sinks to far he starts to drown so he bucks at this point you can't get him back up to the air. Dead horse.
Linked issues
Comments 2
This is not really a duplicate, it actually better explains the problem(13885 states it "sometimes act" as though water does not exist" - the horse does act as though the water exists and floats except for)when you ride into water your are kicked off the horse, you then need to lead the horse through the water and while you are doing so it wants to still feed and will sink about 8 blocks (in creative mode) and then rise, if he goes too deep the rope/leash removes itself. I expect this would make transporting from one land mass to another impossible in survival
Duplicate of MC-13885.