I think its been done with bamboo. That'd be quite a bit stronger than PVC. And you might be able to find it recycled.

And what about tyvek for a skin?

: Hey, I was wondering if anyone could help me out. I need to make a boat for a
: recycled boat race. I'm thinking a Kayak design might win it for us this
: year (other designs have been way to heavy). So i was wondering if anyone
: could send me a schematic of what a basic two person Kayak looks like. The
: plan right now is to build it out of PVC pipe (needs to be kinda cheap
: too) but if anyone has any other suggestions it would be appreciated.
: Either reply to this or e-mail me at glaceria@hotmail.com