A picture or 2 may help, but as Scott mentioned, you want to use 2-part urethane closed cell foam that meets USCG spec's for foam.
2lb./cubic foot density is probably OK, but you can go to 4 lb./cubic foot if you want a bit more strength. It may not resolve your issue with the rough water handling . . . sort of depends on what is making the noise.
Some boats with a one-piece cap/deck/cockpit design have the cockpit sole just laying on top of the stringer grid (not attached or adhered to the grid). In rough conditions as the boat flexes the cockpit sole will 'slap' against the stringer grid causing the noise.
So, maybe check a few things out down below before you sacrifice some storage space.