Skipperjohn's post got me thinking.
Quite a number of years back I took a course in aluminum
casting. We used a fairly fine grained blue styrofoam used for insulation
. Stuff came in big chunks and could be glued, shaped, cut, sanded and so on with wood working tools. We would carve up our models, pack them in sand, pour molten aluminum
down a spout and presto ... toxic gas everywhere as the aluminum replaced the styrofoam in the mold
A person could use this styro to produce a lovely hull
and then lay up glass and epoxy
in and out. I would think it work very nicely for a light weight sailing or rowing dink.