: The only problem would be that the beads would all shift back to the center
: when you got out or water fills up the boat. I say spare no effort for
: backside comfort!

Maybe baffles sewn into the bag (as in a sleeping bag) could help limit the shifting of beads in the bag? A series could be sewn in -- a couple laterally (parallel with the beam of boat) near the back of the bag to support the bum, and several longitudinally (bow to stern) to support the thighs. With baffles, beads could only shift to the center of its chamber. Just a thought.