I know the reason after spending my time teaching black youth at school and asking them this very question.
1. Public pools and their programs are nearly unattended by most black families. This was not so in the 80’s, but lately many black families have shunned the community pool.
2. Many blacks perpetuate the myth that they cannot swim.
3. Remember the Olympics that our beloved media mocked that one African participant in the Olympics? Yeah, that too.
Is this racist? No. I asked my students, and the mentioned each of these answers to me as reasons. One of my kids said “Black folks aren’t made to swim like white folks aren’t made to jump.” I laughed.
I knew black girls who wouldn’t go swimming because the sun made them darker....rare they would go to beach parties unless at night...or if in the day stayed under umbrella’s.