I think it's probably still a demographic fact that most blacks live in either urban or rural areas. Most of these areas do not provide the swim lessons that suburban kids gravitate towards, which are easily accessible and affordable.
My parents grew up in the city, my dad close to Lake Michigan. He wanted a summer job, so he got through the lifesaving course, to become a lifeguard.
My mom, West Side, just west of what was Cabrini Green. She never went swimming. She was scared to death when my brother and I both were in deep water and off diving boards by 4. Thank you dad!
It might be a better generalization than saying 'blacks can't swim' to say, 'many people raised in the cities or rural areas, lack the opportunities provided most suburban people regarding public swimming pools and affordable swim lessons.'