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.
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Also, I didn't realize that undead mobs sink, but after further investigation, I see they do. I still think they should be able to get out of the water to attack.
At least one technically undead mob does swim in water, namely the drowned. However, even that mob has some of the mentioned symptoms, e.g. until player steps in the water, it also just stares and doesn't step up. Though if I remember right, baby versions don't have problems coming out water and wrecking the otherwise peaceful night.
While most of the undead do not swim (i.e. they sink), I also think they should be able to walk on the floor and climb back up, if they have the motivation to do so, like chasing an enemy that got out of water. However, one could argue that their inability to detect things outside the water while (fully) submerged is actually semi-realistic, "total internal reflection" etc. But, they should still detect and react to things on land once their head is above water. So, should be able to get out by walking and jumping, but not necessarily to detect other entities outside of water (if they didn't first see them in water).
I think this should be WAI, since in my opinion, all undead mobz R unable 2 swim.