Yes and no; they were always by the surface or up on land during my visit so I was lucky I guess. I don't know how deep the water is in the middle but I assume they can submerge themselves totally there, or else the size of the body of water doesn't matter.
I guess they are relatively easy to spot if you are not in a rush and have time to wait and observe the water surface and the exhibit. The Pigmy Hippos may be a different story. Their body of water is not much less, but they were sleeping on an island so I managed to see and take photos of them too.
