I helped a friend of mine build a custom car. Since he was in the sign business he used alot of signboard foam to construct the body. he shaped it with a surfoam tool and a r.o. sander. resin bonds too it agressively and it will not warp or deterioate from heat. The car was basically a honda goldwing with the front fork cut off and a frame work attached with a volkswagon front clip. The body was monocoque construction using okume and signboard. It had gullwing type doors and the front end looked like a Ferrari. The only problems he has had with this type of construction was that in some areas where the signborad was under stress, like around the door hinges, the signboard weekened slightly and required reinforcement. It also took awhile to get all of the "squeeks" out, the signboard could be noisy in stressed areas. Thats the part that would concern me using it for a boat. If the construction allows for alot of "flex" the signboard will squeek and croak and will weeken at the stress points.

Hope this helps