Hi Ian. I think you've got an excellent question. There are countless methods of boat construction; most have been around much longer than epoxy. The advantages to modern composite construction would include: durability, low maintenance, weight, ease of construction. There are people out there building birchbark canoes. You'll be taking a significant risk if you invent a brand new method on your own.

Also, consider what is "natural"? Plywood is a highly processed material. You could probably make a kayak like a classic canoe. Your talking about steam-bent ribs (lots of them). Maybe you could make a kayak like an Adorondak Guide Boat? Whatever you do, I would go with a proven method.