The primary ingredient in our boats is plywood, with fir and pine lumber used as well. We call for ACX or Marine Grade plywood, but recommend Marine Grade due to the inconsistent batches of ACX in recent years. The difference in cost when using Marine Grade is not too significant by the end of the project, and you will be confident you have good plywood.
Our boats are plywood monococque structures. This means they're stressed-skin plywood boxes (good-looking boxes, we hope), with the plywood taking the structural loads as well as making the shape. This is a really light way to make a machine, and a lot of airplane designers have enjoyed the sprightly performance benefits of monocoque plywood.
This method of building is not at all as complicated as it sounds! The plywood panels are held together with stringers attached with screws and glue. These stringers are ripped from fir stock, but some people have substituted other medium-hard woods such as spruce. The bulkheads, hull bottoms, decks, transoms, and keels are marked with centerlines as you build. If you keep these lined up, the boats come out straight and true at the end. There is no need to make jigs or bucks to help keep the boat straight while building; these boats are designed to square themselves up if you keep all of the centerlines of the panels properly aligned.