It’s amazing that just about anyone can lie on their back and float.
When I skin dive, I wear weights so I can stay below without exerting energy from to keep from automatically floating.
Ditter, when I was younger I was really dense (no wise cracks, please!). I could lie on the bottom of the pool. I did not float.
While at Georgia Tech all freshmen were required to take Swimming/Drown-proofing. This course is the basis for the USMC Drown-Proofing course. In it, we were shown that we could stay afloat for at least an hour, even if we were sinkers. We were also taught to stay afloat if injured. Arms tied behind our backs or legs crossed “Indian-style”, tied and the rope tied around our waists, to simulate broken legs.
Even trussed this way, we were able to swim across the pool underwater, swim two laps on the surface and dive to the bottom of the pool and recover an object. To earn an “A” in the class, our hands were tied behind our backs, our ankles were tied together and we had to swim from the shallow end to the deep end, bob for 20 breaths and return to the shallow end.
Even I, a sinker, was taught to accomplish this. You can stay alive in the water, even when injured, for a long, long time, if hypothermia or “critters” don’t get to you.
I could not lay on my back and float when I was younger. Relaxed, head up and breathing, I would slowly slip under the water.