Having purchased the boat last year and the wife complaining about the "level of comfort" we have just had our cushions re-done by an upholsterer. What the wife had been sleeping (and we thought were okay) turned out to be 2" vinyl-covered rubber cushions that were so far gone it was actually more comfortable slleping on the plywood bunk base!
After several trips from his workshop out to see us and much discussion over thicknesses, fabrics, foams etc., instead of the "traditional" 4" soft foam, we finally agreed on 3" high-density foam with a cushion-backed velour (car seat fabric) cover as being cheap, but comfortable.
The foam he used was interesting stuff (apparently there have been many advances in "foam technology" in the last few years). The new foams allow water to "pass-through" and the velour allows the cushion to "breathe" - something which the old vinyl didn't, as demonstrated by the amazing amounts of mildew inside when he ripped open the covers.
Hey, it might not last 20 years.. but hopefully we'll have a bigger boat by then!