I totally agree about needing both a core material and glass on the outside.
I built a solarcar from carbon fiber in college. You need both. We used prepreg, where the epoxy is already on the carbon, and it needed to be heated in an autoclave to get it to kick. This let us use a honeycomb core, nice and light, but unless you have access to one, foam core is the way to go. You can strip plank a boat out of foam core, the same way kayaks are done here. I saw a dinghy up at the Maine BoatBuilders show, I swore it was wood till I got up close, real sharp

A book on building with foam coam can be bought here:
http://www.atc-chem.com/About/Newsletter_list.asp

CoreCell supplier http://www.atc-chem.com/About/Newsletter_list.asp