I just noticed this thread...

I'm currently building S&G greenland kayak from foam core. The main reason for using foam instead of plywood is closed cell structure where epoxy coating and holes are no problem. I feel that working with plywood needs a lot's of accurancy to keep every bit well saturated with epoxy. I'm not looking for major weight savings but there should be some. The stucture needs more fiberglass for punture resistance but surface will end up tougher. I tested the fiberglassed panel applying my whole weight on a small nut - I couldn't do a dent to surface! Plywood with light glass would have serious dent there. Foam could be used for more curved shapes - altough mine is pure hard chine hull. There's grooved foam sheets available that will have more flexibility, it can be used for more curved surfaces.

In other hand, the foam is more expensive, epoxy and fiberglass costs more and in some ways the foam is more difficult to work in fiberglassing. More fiberglassing means also more worktime.

My first kayaks were S&G plywood - I might do a kayak in that way again. However the foam core has some clear advantages so it will most likely be the choice for long lasting kayak.