From here http://www.swimmingwithoutstress.co.uk/swimming-blog/black-men-cant-swim
…"Matt wonders, Is it true that black men are not ‘made to swim’ physiologically?
Matt Bridge, Senior Lecturer in Coaching & Sports Science, University of Birmingham
Expert on human movement and human physiology.
In terms of buoyancy there are some differences and there is evidence to suggest that black men and women are less buoyant than white men and women.
Differences in bone mineral density (weight of bones)
Take average black man and average white man and you’re looking at 300grams difference in terms of skeletal mass.
But there are people in both groups that break the rule, you can get very buoyant black men and very heavy white men.
But generally black people are less buoyant than white people.
But a non buoyant person can still swim.
Lack of buoyancy is most significant at the learning phase. e.g black kid kicking with float, legs may sink. Once you take the floats away, it evens itself out.
Well, according to this article, I think I sort of guessed right. It’s a matter of density. And regardless of which body part of blacks is more dense to me, from an engineering standpoint, the main part of swimming is staying afloat. If you don’t have to work as hard at floating that energy can be directed toward forward motion. And that’s really going to make a difference in longer races. This in my mind corresponds to running races where whites and blacks are more even the longer the race gets, since sprinter just need speed, distance runners are fighting fatigue.
A second non-statistically or scientifically confirmed (by me) observation on my part as to why whites might float better is that for equal height, blacks seem to have longer legs and shorter abdomen than whites. In my tiny mind’s eye I see abdomens as having less dense non-bone tissue that floats better in water, but it’s non-propulsive weight on land.
Maybe I’m wrong, maybe it’s just a matter of fewer blacks swimming because of cultural or economic reasons and if that’s ever changed they will do as well at swimming as they do at land sports. But even that in my mind will show that there’s a physical difference between blacks and whites as in my mind, blacks make up a higher percentage and have better records in college and professional sports and than the general population.