Can everyone float?

I tried to float, and learn swimming since I was a kid, but I only sink downwards.

Even with lungs full of air and in a horizontal position, I am fully submerged.

Trying to understand the physics of it, and whether it is truly possible for all human beings to float and be able to breathe in fresh air.

Also, does the orientation of the body matter? I notice we tend to sink instantly when upright, but have a better chance when horizontal.

Is the neck and head truly less dense than the rest of the body? (Lungs are fully submerged in both scenarios, so leaving that out.

For context, I’m 5’3” and 108lbs F