This is silly - even if it was true that "black people tend to have more dense fast twitch muscle, while white people tend to have less dense slow twitch muscle

This is silly - even if it was true that "black people tend to have more dense fast twitch muscle, while white people tend to have less dense slow twitch muscle" (and I doubt it is) its not relevant. All people who are not physically handicapped can be taught to not drown, and that's what we are talking about here, "not drowning." We aren't talking about competitive level swimming, not swimming as a sport, and not even recreational swimming, we are talking about "stating alive if you find yourself in water." The fact that most (~70%) of black people lack this skill is a serious public health issue.