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.
Oh I know...I’ve done the drown proofing during lifeguard training....with people trying to fight you in the water...being comfortable in the water from an early age is key.