Extinct now. They bear a thin skin and the speed of water loss via the skin (dry air) is much greater than that of the rest of the mammals in the world. The Madagascan Dwarf Hippopotamuses have to be in the water most of the time if the location is humid. Otherwise, they would simply get dehydrated.
A Madagascan Dwarf Hippopotamus had stubby legs, and as it ages, these legs would be less and less capable of supporting its great weight. So, when these hippopotamuses reach a very advanced age, the water's buoyancy is crucial in lifting themselves up.