Polyethylene boats can be recycled if they are constructed of linear polyethylene, either low density or high density (sometimes called super linear polyethylene). Cross-linked polyethylene cannot be recycled. I am not sure about the material used for construction of the 3-layer boats.
I don't know of any manufacturers that receive recycled kayaks at this time. Polyethylene boats must typically be recycled through some local center. Oft times all non-polyethylene material (metal, foam, etc) must first be removed and the boat sawed up into pieces below some maximum length limit.
Dagger kayak has manufactured kayaks made of recycled polyethylene. These boats are typically recreational models. The recycled plastic is not as strong due to consumption of anti-oxidants in the material at the first molding. My guess is that recycling plastic would not be too attractive to boat manufacturers as it would entail additional machinery and shipping costs that would probably outweigh any savings over not having to purchase polyethylene powder, even if the recycled plastic was received "free".
Here is an extensive pdf regarding manufacture and recycling options for linear polyethylene boats if you are interested:
http://www.wspenvironmental.com/media/d ... atible.pdfI am not aware of any ability to recycle either ABS or composite boats. I have repaired quite a few old and broken boats partly to keep them on the water as opposed to taking up residence in landfills.
As for a waterproof, non-toxic coating for wood are you overlooking fiberglass? A waterproof and virtually invisible layer of fiberglass can be applied to the exterior of most boats of wooden construction using either epoxy, vinyl ester, or polyester resins.
I don't know too much about vinyl ester resins but I have read that they are attractive to boat manufacturers because they cost less than epoxy when purchased in bulk and vinyl ester tends to penetrate multiple layers of cloth simultaneously better than epoxy does, which is important for boats made using vacuum bag or infusion methods. Vinyl ester is less attractive to home boat builders and amateur boat repair guys like me because of greater toxicity (due to the methyl-ethyl-ketone-peroxidase catalyst) and a much more limited shelf life. Epoxy is also said to be somewhat stronger than most vinyl ester resins, although more expensive.