I agree, a buoyant pack would be problematic in many ways -- especially in fast-moving water. Good advice too regarding testing and practice, since if you're tightly strapped in the pack could tend to hold you face-down in the water (yikes). I know that backpackers normally undo sternum straps and hip belts before fording mountain rivers.

The bottom line is that there is no good substitute for a PFD. But how many have one in their car?