While that ratio may be a benefit in running, it's more of a negative in swimming, performance-wise.
I'm reminded of this everytime I scuba dive. I'm not black, but I'm well over 6 ft with very long legs and low body-fat. I have to "trim" in order to keep my legs from 'sinking' so that when I fin I remain 'level' in the water.
Long distance swimming is particularly tiring since I have to exaggerate my body position in the water. I kind of have to tuck my chin to draw my body's center of mass more over my center of buoyancy. This allows me to propel myself through the water more efficiently. If I don't my legs 'drop' and some of my motion is required just to keep myself 'up' in the water. Much more tiring.
I’m caucasian and was a sinker when I was young. Too skinny. I was actually a good swimmer, but I just could not float which caused me to have to quit a life-saving class. Any time I tried to float I sank to the bottom like a stone. Sadly, I could float a lot better now fifty years later.