A few years ago, my wife worked on an ethnographic research project about the lack of swimming skills in the African-American population. One thing that was striking was the difference with other multi-cultural countries --e.g. the UK and South Africa-- where there are no such differences in the acquisition of swimming skills between ethnic groups. In the UK, learning to swim is part of the national curriculum, unlike the US [edit: I don't mean this to be an exclusive explanation].
On a personal note, the notion that so many US children don't know how to swim is horrifying. I consider it a basic life skill, as important as knowing how to cross a road. My daughter learnt to swim before she learnt to ride a bike.
[1] http://www.bbc.com/news/world-us-canada-11172054
[2] http://well.blogs.nytimes.com/2013/06/10/swimming-and-the-fe...