My own feeling is that a waterproof pack, or a pack with waterproof bags inside, will float. Particularly if you dedicate a single dry bag to serving as a flotation bladder inside the pack. Of course, you'd want to test that proposition before giving it a try under real-world conditions. Otherwise, you could always find a way to attach some sort of flotation device to the pack. Wrap it in a kayaker's PFD, for instance. Anything that can't be punctured, but floats, could be clipped or lashed to the exterior of the pack.
Anyway, hope that helps. Perhaps some readers who have tried the same thing could post their thoughts in the Gear Forum.
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