When a mob that has a passenger mob riding it becomes submerged underwater, the rider will be dismounted and the two will be separated
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Comments 3
I had tested this with Creeper riding Zombie, Creeper riding Iron Golem, Armour Stand riding Zombie, Ravager riding Phantom and Skeleton riding Spider, the last was the only one that didn't dismount, probably because the spider floated on top of the water
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We're tracking this issue in MC-72440, so this ticket is being resolved and linked as a duplicate.
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That's definitely WAI with horses, other mobs I'm not sure