Brian,
I finished the deck of my Merganser with Minwax Helmsman Spar Urethane and am very pleased with the results. It goes on smoothly and sets up quickly, but can be touched up for a few minutes, which you cannot do with varnish. It dries to a sandable and recoatable finish in four hours at 72 degrees. The final finish is harder than, and about as shiny as, varnish. Minwax claims it has the same UV protection as spar varnish. Best of all, it costs $16/qt, vs. $37 for Interlux Schooner varnish. - Wes