hi i have a 1988 starcraft 18ft aluminum v bottom. i am replacing the floor and found foam was a little wet. it is only wet on the bottom but is still a little heavy. just wondering if there is an alternative to expanding foam since it is so expensive, like maybe styrofoam board with moisture resistance? or is the foam act as a structural part of the boat? the foam only runs down the sides of the boat, the middle section is all open for the gas tank and ski storage so i wouldnt need much i dont think. if i decide to do the foam would the 2lb foam work?