There is a difference in using a pack or any other floatation device (in front of you), even a PFD to float in a controlled condition and another when you are suddenly plunged into deep water or swiftwater. I wear a rescue PFD that provides 23 lbs of floatation, if I jump into the water; I submerge for 2-3 seconds before coming to the surface, which is no problem because I am expecting the initial shock of being submerged. Additionally in rapidly moving water and depending upon your profile (with gear), the increased surface area can act as a wing or plane forcing you down. It is a big no-no to wear a fire helmet in the swiftwater environment, since the large brim on the back with act as a wing, which when hit by moving water will force you underwater. If you look at kayaking helmets or water rescue helmets, they have little or no brim and usually have vent holes to allow water to escape from the helmet. A backpack worn on the back and hit by moving water would likely have a similar effect and force you down. I would agree that a watertight bag or backpack with trapped air held in the arms in front of you would be a useful floatation device, but if you are wearing it on your back be prepared for an unpleasant experience. I would suggest trying it out under controlled conditions, so you can see the dynamics before you trust it during an emergency.

Pete