Plywood

It's okay to use plywood for your forms. Plywood does tend to warp which can lead to a distorted hull. Generally, distortion problems can be corrected but it's a good idea to keep a discriminating eye on the lines of your boat and check it's alignment periodically. I string fishing line above the keel to help eyeball the alignment.

If you are using plywood for a stitch and glue kayak or for making other kayak parts, the best grade is marine okume (also spelled okoume). This is an expensive, certified marine-quality plywood that is quite thin. It is rot resistant and the laminating glues are waterproof. It is usually available in thicknesses of 3, 4, 5 and more millimeters. For cheaper construction, some folks have used "mahogany" door skins. These are cheaper plys that are made of wood that looks sort of like mahogany and is sold as a replacement for now-hard-to-get mahogany. Higher quality, but not waterproof plywoods that are available in thin versions include Baltic birch plywoods. These come from Finland and Russia, can be furniture grade and are not outrageously expensive. Unfortunately, large sizes are harder to find and Baltic birch is often sold in sheets about 4 foot square.

If you buy conventional plywood, you'll find that the glues to bond the laminations will not hold up well in water. Cover with a waterproofing varnish and used at your own risk. For the most part, conventional plywoods are only good for dryland tasks like making forms.