772 children in six U.S. cities by USA Swimming and the University of Memphis revealed that six out of every 10 black or Hispanic children couldn't swim.The survey also found parental influence was the largest contributing factor in his or her child's interest in swimming. Minority children, the survey states, are six times more likely to come from a family where one or both parent haven't learned to swim.
Swimming is a learned, and reinforced, activity. Comfort zone comes from repetition. If your family can't afford to have a pool, doesn't live in a neighborhood that has a pool, doesn't live anywhere near a swimmable body of water, or has finances tight enough to reduce or prevent visits to a community pool, then that activity is not going to get reinforced.
I suspect that there was a time where MOST city dwellers were not good swimmers, mostly because of lack of access to a body of water conducive for swimming. Again, this is class, not race.