Flotation

The deck of the new kayak needs some ridged flotation. The main reason for this is to deflect water that is sloshing around inside the hull away from the gunwales at the top. I have noted an unpleasant effect in big waves when the kayak has taken on water through a battered-in spray-skirt. If the seas suddenly roll the hull from one side to the other, the bilge water launches itself as far upward and out-board as it can. This can easily pass the tipping-point and over it goes. With ridged flotation the inside profile of the kayak can be shaped to deflect the surging bilge water inboard and down.
The trick now is to find insulation that is bio-degradable. Styrofoam is no good. Cork would work but it is costly. Papyrus has been suggested. What else?