I've watched OW students do their treads for five years now, and I'm amazed at the range of buoyancy of a bathing-suit clad body. I have seen students who were nearly exhausted after ten minutes, because if they paused kicking for one moment, they instantly sank. I know from my own experience that I have trouble getting underwater even with empty lungs -- I float like a cork. It's the ratio of bone (which is negative) to fat (which is positive). A big-boned, muscular man with no body fat is going to be somewhat negative, and an osteoporotic woman carrying a few more pounds than she ought to is going to float.
Legs will always be more negative than the torso, for the fairly obvious reason that the torso contains relatively few bones and quite a bit of air.
Legs will always be more negative than the torso, for the fairly obvious reason that the torso contains relatively few bones and quite a bit of air.