When I was in the military, we did team PT and decided we’d teach the black guys on our team to swim. The encouragement just wasn’t there when they were younger to learn to swim, maybe due to cultural differences?? We started with floaties and gradually moved up to get them over the fear they had ingrained into their families of the water...When finished they were so happy to have that basic skill. Hopefully, since then they’ve encouraged their family members to learn that vital skill.
It is a cultural thing, and it can be overcome. But probably not to a point that you could take your superior black-athlete, non-swimmer, and get him ready for SEAL/BUDS. It's not just about getting a person to learn to swim to pass the swim test. It's about 'comfort' in the water. That starts from being exposed as a kid.
The stuff that they do to you in the pool at BUDS is designed to freak you out. You have to be super-comfortable in the water to overcome your basic instinct... to KNOW that you can recover that regulator. I've had a small dose of it in advanced civilian SCUBA training. And that bar is set low compared to what these guys do.