This one is actually true. Athletes (and I'm assuming these guys are DBs or something and not fat *** linemen) have a much lower body fat percentage. Fat floats, so because they have less of it, the float less easily. The bone density thing is correct also. It's been shown that African-Americans have a greater increase in bone density after puberty. Asians actually have less increase than your average whitey. Of course, they were studying racial differences in bone density to figure out why osteoperosis was rare among black women, but denser bones means really do mean you float less easily.
However....
This is all well and good when applied to getting in the pool at your local YMCA. When miles away from shore, drifting in the cold open ocean with 9 foot swells, being a fat white guy isn't going to matter in the slightest.
/Really, it's all whether you may more or less than a duck.