I sometimes exhale and lie on the bottom of a pool for a few seconds.
Without air in your lungs, you sink. Once you start decomposing you
will float. An alternative to decomposing is just to keep air in your
lungs. That's what most of us do.

The only way I can float is to keep my lungs 100% full. 70% and
I'll start to sink. It makes it difficult to tread water when you have
to expend more energy just to breath.

Bone density, fat percentage, and muscularity determine your buoyancy.