A friend of mine makes Swampscott Dorys using rigid PVC foam. He makes the boats upside down on a frame as is done with plywood. He then glasses the bottom. Temporary frames are then attached and the works flipped over. The inside frames are removed and the interior glassed and the gunwales attached.

PVC foam is superior to urethane foam in mechanical properties and resistance to delamination. Last-A-Foam is a bit better than the pure Urethanes, but not much. We use the higher densities as a tooling foam since it is easy to machine to a good surface finish.

For my money, I would use PVC (Divinycell) rather than Urethane or Last-A-Foam for a boat project.