Dunno about this. Percentages might be right but I surfed with and competed with black guys , even had a guy shape me a board. One guy was 6’-7” and used to be on the Lakers bench back in the 60’s. Couldn’t surf all that well but he sure loved the ocean. I suppose this has to do with not many blacks living near the ocean unless it’s Africa.
I dunno, at the L.A. beaches there were plenty of blacks, as they lived just inland and an easy drive.
That said, we used to laugh how they never went any deeper than up to the knees. Never actually saw swimming going on, though there must have been a few. Just a lot of splashing around.
Rivers, lakes, and the ocean all have some pretty nasty critters inhabiting the waters in and around the continent of Africa ... excepting the Med to the north. Not much incentive to be frolicking in those waters.