I have made these boats using only water-based polyurethane to seal them. This has the advantage of slightly lower cost, water clean-up and lower V.O.C.s. Dries quick, too. My first boat was done with this, and its has stood up very well.
More recently, we have been using Waterlox, a commercially available, tung-oil and resin wood finish. I like to use two coats of the 'origianal' type, followed by a top coat of the marine version of the same product, which, rather than soaking in and filling the grain, leaves a more varnish-like gloss coating on the surface. It takes a while to thoroughly dry, particularly if there are thick spots or drips around the lashing, etc. Brush it out as thin as you can, and give it several days to dry, preferably in the sun.
The most efficient way to do this is to have the boat frame, the two rub-rails (for the gunwales) and the assembled floor boards all done, so you can finish them all at the same time. With a few home-made 'pin boards', it is possible to coat both sides at the same time.
