When I was in my teens I worked as a lifeguard on an Army base. Part of our job was to do “drown proofing” with the soldiers. Almost every black guy would go straight to the bottom like a rock and end up trying to crawl up on your head when you brought them back up LOL!! In general they seem to have very little body fat so part of the problem was they had zero buoyancy. I think a lot of it comes from exposure and being comfortable in water. What a tragedy for the mom to lose 3 children. Can’t imagine.
Passing a formal drownproofing class used to be a requirement for graduation at Georgia Tech.
http://en.wikipedia.org/wiki/Drownproofing
They dropped it in the mid 80s, several years after I graduated.
There were people who just didn’t float very well using the technique.
My three kids are “sinkers.” They’re all adopted and all Asian. They also have no fat.
I never could teach any of them to float the way my mom taught me. We gave them swim lessons, but it was a struggle for the first two to learn the lessons in the order presented.
But they all swim now. My youngest somehow figured out how to swim underwater and that was the key for the older two learn. Maybe sinkers need to learn to swim underwater before they get up some enough speed to learn to swim on the surface.