I made a gorgeous hippo lake with an underwater viewing area for guests. I was so proud of it. It was the first time I felt I got the rocks and weeds looking right in an underwater habitat. Then I placed the hippos and found out they didn't have enough space (even though they should have). Turned out they can't traverse surfaces with rocks or plants on the bottom of their habitat! Hippos "walk" underwater rather than swimming, and for whatever reason the game feels they can't walk on rocks or plants, even if they are in water.
I was bummed! Had to pull most of the underwater decorations out of the habitat. So be careful with "furnishing" underwater habitats. This is less of an issue with animals that only swim, of course.
Some people have done some nice things with some of those leafier water plants by stringing them together to look more like kelp or taller water weeds. Wish they'd included actual kelp with the North America pack, though. Would be more realistic for the sea lions, and the color of the fake kelp I made with those Asian water weeds is too green, and of course the plant itself lists as not appropriate.
You can also make fake coral with those underwater branch thingies and coloring them appropriately.