Zombie villagers can't drown in water - their "Air" value remains at 300s. When I modify this value to any number, it doesn't change either. Even negative values don't cause the mob to take damage.

This issue has been tested by me on zombie villagers:

• spawned naturally (savanna type)
• spawned with an egg
• spawned with /summon
• which has been a savanna-type villager who was infected by a zombie (both mobs spawned naturally)

and in every case, it works as I explained