Originally Posted by Arnox

They don't swim because on average, they have denser bones than white people. This makes them much more fitting for running, but means that they sink easier than non-black individuals.

There's some discussion in the scientific community, and it's entirely possible that it's a result (at least, to a small degree) due to slavery. Blacks were used in an extreme way and only the fittest, strongest slaves survived to pass on their seed. This also reflects upon the nature of living in Africa - very little water, a lot of pressure on being as efficient as possible with sweating and remaining cool while on the move.