Evolution throws up strange results that leave us struggling to make sense of it all. For example, we know that hippos and cetaceans (dolphins, whales) evolved from a common ancestor, but it's not exactly obvious - more counterintuitive than not, on the face of it.
Some insight can be gained by watching the hipppopotamus underwater. They are far more graceful in water than they appear to be on land, which demonstrates the extent to which their body has adapted for semi-aquatic living.
A couple of scientists - including the interestingly-named Frank Fish - have studied the action of hippos under water, and published their findings in the Journal of Mammalogy (reported in New Scientist). Hippos don't swim per se: they move forward by "punting" - touching one or both front feet to the riverbed, and pushing forwards. For this, the weight and bone density of hippos are well suited to the bouyant aquatic environment.
The authors liken this to the microgravity environment of an astronaut, although they then put it more in an evolutionary context.
Dr (or Mr?) Fish suggested this gave a clue to the development of swimming in cetaceans: initially, "they may have walked on the bottom and foraged".