I've seen a couple of zombie villagers underwater that don't swim and that aren't drowning. Not sure if they spawned there or wandered in; I can only confirm 2 sightings thus far near a neglected village. No regular zombies (or any other mobs that don't belong) as of yet but not sure if that's just coincidence.
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However, this issue is a Duplicate of MCPE-33219.
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Allen's comment made me think I should double check the wiki and I now see that zombies sink and do not take drowning damage by design. I had missed that tidbit when Aquatic dropped so there probably aren't any issues here.
I will say that I noticed that a zombie villager appeared to spawn underwater, and not sure if THAT was by design (was in the middle of an ocean with no land in sight), else I'm guessing this can be closed.