Whereas urethane type foams (klegecell, airex, divinycell, etc.) are great foams to use, they are also very expensive. For various projects (NOT HULLS), you can use styrene foam. We get stuff that is 3 lbs/ft2 density that is pretty good and VERY cheap......about 1/10th the price of urethane foams. You can ONLY use epoxy resins with this kind of foam. Polyester will melt it immediately.

If you are going to use it as a "deck" or sitting/walking surface, then point loading problems can be solved by bonding 1/8" plywood to the top surface. A typical layup would be (from top to bottom) 3 oz. glass, 1/8" pl;ywood, 1/2" to 1" styrene foam, and then 10 to 18 oz. glass on the bottom. We sand bag or vacuum the plywood to the foam and do hand lay-up for the glassing.

It's will be light, stiff, inexpensive and strong.