Interesting story. A pack that traps air woulld be a good idea in such a scenario. Wrapped tightly in a poncho it would likely float. And perhaps there was some box, keg or other item on deck to perform as a makeshift float, tethered to the pack even if it was submerged. I have floated a fifty pound Alice before. Pack liners are handy in wet environments. Few packs weigh as much as the water they displace. I would attempt to get away with it all and still be willing to lose it all rather than my life. After all, you were not there alone in that situation.