I didn't get it the first time I read it either. Since this is racism, and stereotypes, "black" people are good basketball players, so all the white guys in NBA apparently sit on the bench.
As a black person i can confirm the whole swimming thing. Black people don't swim. We also don't go camping, ski, skydive, chase alligators, do our own stunts, jump off roofs into snow banks, spelunk, go on big vacations, serve diet items on thanksgiving, or have labradoodles or chihuahuas as pets.
As a black person that lived in coastal Georgia for over 9 years I disagree with the no swimming thing. My family and I swim every chance we get. Maybe we're just weird tho.
And that's my cue to leave