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.
[QUOTE=Rosalind]
Aren’t all human beings denser than water? I don’t mean just those who have fewer brain cells than a bottle of Perrier.
[/QUOTE]
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.