Decision: Composite

I found some foam, and it was a reasonable price, as was the delivery. Also some very reasonably-priced (for here) meranti marine ply, ¼".

"A sandwich construction of a core material faced on each side with a reinforcing glass/resin skin offers greater mechanical strength and stiffness, pound-for-pound, than any other structural option, regardless of the materials used."
-http://www.compositesworld.com/articles/wind-foam-sources-pet-san-pvc

I'll make the bottom and—since I intend to install a rudder—the transom, out of the ply. The rest of the hull will be foam/glass. The foam is PET: polyethylene terephthalate. I'm using 10mm (⅜" equivalent) for the hull, and 6mm (¼" equivalent) for the decks. The "composite" part of my post title is about using both plywood and foam. I want the plywood on the bottom as insurance against harsh groundings on sharp objects.

Here are the two bulkheads (green foam) and the temporary forms. I haven't cut the transom out yet. It would sit on top of the left stack:

Below: Laying out the bottom on the mahogany ply. That single sheet of ¼" ply was heavier than all four sheets of the foam, which of course won't all get used because of offcuts.

The bottom is over 8' long, so the after end gets a bit scabbed onto it:

I left the drawing of the final shape of the after end until after the whole length was there, to make sure the curves are fair between the two pieces. The after end is only 1" wide!