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.
"A knife in a mans hand is as precious as a diamond necklace on a womans neck" Felix Mirando
As the great Andrew Martin once said "One is glad to be of service".