Actually i saw a article on why black people are rubbish at swimming and good at running. Basically for running, you need long legs and short body, to get a high centre of gravity, which helps as you basically 'fall forward'. For swimming, you need a long body and short legs; the opposite, as legs just get in the way, and most of the power comes from the arms.
Genetically, white people tends to have the latter body shape, and hence are better at swimming. Black people tend to be of the former shape and are better at running.