Zoglins sink, they don't take drowning damage anymore, but they still sink when put in water and when mobs are on land, they can't seem to climb up the blocks in the water to get on land. 

The pictures are to show that they aren't moving, they just stare into the block in front of them. It's not until the mobs go into the water with the Zoglins that they can really move and kill them.