The largest internal migration in American history was that of African Americans from the rural South to the industrial North. As a resident of the (post- at this point) industrial North, I can assure you that the average city kid of any ethnicity doesn’t get many swimming opportunites. This is doubly true as the shore points in this area (South Jersey) are white working class strongholds in the summer, and even though I imagine the black middle and working class could afford at least that in some cases (keeping in mind a lot of poorer white folks can’t either), there have been racism problems over the past several decades which would be more than enough to keep people from making that trip.
Hell, I’m a white city kid, working class background, and I never learned to swim very well. My parents and grandparents grew up in the city & can’t/couldn’t swim either. And that’s even ‘being allowed’ to go to the Jersey Shore wihout incident.
Add in public pools that are frequently empty due to budget problems, parents not knowing how to swim either, etc., and you have some big barriers in the cities that I don’t think would be so much in a rural area.
Similarly American Jews dominated basketball and did quite well in boxing when those were the only available sports in the city gyms and YMHAs, back when working class Jews were packed into the cities. Nurture is huge in sports.
Now, the following is some rather extreme speculation on my part and I’m not sure how much if any of this has carried over, but it is fascinating:
My ex-girlfriend, an anthropologist, lived in St. Louis, the second largest city in Senegal, for a number of years. Apparently, although on the Atlantic, and although many people make a living fishing, very few people can swim. More amazingly, lots of people living even within a half mile of the ocean have never seen it in many cases! That would require crossing a river, which is traumatic enough for some without contemplating as much water as the ocean. When she took some women to see the beach for the first time in their lives, some of them became woozy and disorineted by the vastness and open space, and were very much afraid to wade into it.
Among other things, this made me aware of how additionally traumatic the crossing involved in the slave trade must have been for certain populations (on top of all of the other problems).
I don’t know if any of this has survived in cultural memory for some people or not, but I know from personal experience that one generation not knowing how to swim can be transferred to the next easily until someone makes a conscious effort to break that chain.