I'm a WSI and Lifeguard (Red Cross and BSA) - but if I stop swimming I sink. It all depends on things like muscle mass, bone density, etc. Not as much on body fat as you would think (I was a sinker at 16 and I'm a sinker at 55, even though I weigh 30 pounds more). But some folks just float like corks, and others head straight for the bottom. If I cram my lungs full of air and keep my hands moving I can float, but just barely . . . but the Drownproofing thing still works for me.
Even as skinny as I was growing up, and even as an adult, I had to fight to get to the bottom of the pool, even in water only 4’ deep. I gave up trying in my 30’s and became content that if I died in the water, I would be found easily.