I'll touch this.
I mean, I don't know but I'd bet it stems from segregation laws prohibiting black people from using pools which produced generation after generation of people who never learned to swim resulting in a fear of water considering any kid who has a general knowledge of how to swim is still at a high risk of drowning. Saying 'Black people can't swim' is just another means of drawing attention to racial differences as there is a higher percentage of black and latino vs. white in America that have "low or no swim ability" (according to a 2010 study by USA Swim) - because when you look at the small picture, go to the public pool or beach, it doesn't seem like a big difference...but then when you look at the grand scale and start making statistics it seems like a much bigger difference and tah dah you have a tool for.... yaaaaaaay racism!