I used to be a scuba (SCUBA) diver, and yes, that is not so incorrect as you imply. See my post a little higher suggesting that the bladder could contain a cou

I used to be a scuba (SCUBA) diver, and yes, that is not so incorrect as you imply. See my post a little higher suggesting that the bladder could contain a couple cubic feet of air, which would give you a couple-three minutes. The real problem is diving, rather than floating on top of (or more likely underneath) the bag, as others have pointed out.