Some of us literally sink if we don’t swim.
One time my friend looked at me in awe as I laid on my back, relaxed, and sunk. We tried it over and over again. I was as curious as she was. She had to put her hands under my body to keep my body above water. When she removed her hands I’d begin to submerge, just laying on my back, arms stretched out, until I was fully underwater. But she did the same and she would float.
I wasn’t tense, but rather relaxed and not panicking, since I could swim (the only way I know to stay afloat) and knew she was right there. And I mean, not that hard to just lie there calm and relaxed. I have to actively swim in order to stay afloat. And I’m not the only one..
Although strangely enough dead bodies float so what’s up with that?