Don, I have used a tung oil / polyurethane mixture available here in Australia from paint suppliers, for deck planking on several of my models. It is somewhat thinner than straight polyurethane, and seems to penetrate the timber very well. I usually apply 3 coats, and I'm very happy with the results. It is also very water resistant, as water just "beads" on the surface and is easily wiped off at the end of sailing.
Peter.