I really hope that animals other than hippos dive underwater. Crocadilians spend so much much time submerged it would be strange for them not to in the game. It also makes underwater viewing areas better.
Same goes for surfacing animations. I'd love to see the hippos surface to breathe from time to time.
Not necessarily as a mechanic (drowning) as it might come with an entire set of problems of its own, but more as a cosmetic/visual detail.