You may be right .Feathercraft kayaks has been using HDPE frames for years , but they sell folding kayaks , so if a frame gets bend , you can take it off , warm it and shape it back
The material is cross-linked polyethylene and specific density is around .95 . about the same as ebony but plywood is around .6 - it would add a couple of pounds to the kayak . plywood frames may be a better choice . frames don`t touch the skin so trapped water is not a big concern (stringers do trap water ) .
http://feathercraft.com/frame/