Unlike skeleton horsemen, zombie horsemen dismount when going underwater, even though both the zombie and the zombie horse are undead. To reproduce, run the following command near a river or another deep pool of water:
/summon minecraft:zombie_horse ~ ~ ~ {Passengers: [{id:"minecraft:zombie",equipment: {mainhand: {count: 1, id: "minecraft:iron_spear"}}}]}When they’re tempted or forced to go underwater, they will dismount.
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