Sometimes the animals won't go near a particular area or object because there's no reason for them to want to go near it (a bit like in real zoos I suppose). If you haven't already, you could try putting one or two of their enrichment items into the water to encourage them to take a dip (toy enrichment would probably be your best bet).
It could also be that the water is too cold for them. If it is you'll need to place down a water heater nearby (although I doubt that's the issue, seeing as you're on a grassland map if I'm not mistaken).
Or it may be the structures you have inside your water are so high that the hippos can't pass them. It is a long time, I had hippos in my zoos, but if I remember correctly, they kind of "walk" on the ground when in water. Maybe they can neither walk no swim around/above these obstacles.
Edit: and the tree under water might block them on the ground, too, although it does look as if they can swim above that.
I have underwater grass but I don't think that would cause any problem.
I also have an underwater viewing tunnel with a walkway over to allow the hippos to walk over it. Maybe the problem?
I think because they stay on the water border and don't go in, it might be an issue with the water level or the steepness...
I will do more testing and see if I can fix it...
Did you try to delete the tunnel? (Just try it - you can use "undo" after you saw your hippos use the water or not).
After looking at the pictures again: Perhaps it is the part next to the tunnels and the grass, which is too steep? What about the environment-tab in the animal menu? Does it show enough water?
The traversable area map shows that it is accessible, neither blocked nor too steep, if that were the case it wouldn't show up blue.
Several things:
1. Does the info panel show the water need as met or not? If not either the heatmap or the animal is somehow bugged. In that case try to reload the zoo and see if it fixes the issue.
2. Did you already try to reset the habitat? Move the gate a little or add new enrichment ect. to force the game to recalculate the area which can fix issues with navigation.
3. Before you completely redo this habitat try moving the hippos to a new one with nothing but some water in it and check if they go into the water there. If so it's something about the set up, if not it's probably a bug with the animals rather than anything else. Then you can try to verify the files via steam and see if that makes the lazy Hippos finally go take a walk in the water.
Figured out the problem! I noticed my water filter showed 2 water bodies being filtered when it only should be 1.
Looked under the map and noticed a very tiny pool of water that could only be seen underground and deleted it.
This small pool somehow stopped the hippos from recognizing the larger pool. Anyway my hippos are back in the water and back to normal.