Skinny little college chick here, add me to the “can fall asleep floating” pool. 
…in any position, too. In addition to the “face up” and “dead man”, I can susain a watery “armchair” lean for forever.
Insane boyancy has been my enemy in retesting for lifeguarding summer job, though.
Check out the densities for fat and muscle that Santo Rugger gives in this GQ thread. Bone is also denser than water, of course.
So it’s mostly just a question of whether you have more muscle+bone than fat. Lungsfull of air can also alter your overall body density.