Some of that wood preservative stuff that's for decks and patios. Everything will break down and have to be re-done about every year or so anyway, even the spar varnish.

We used to use a product called "wood life." Not sure who it was made by, but it was great stuff. You could get it at Lowe's or Home depot. Just be careful to wear eye protection...that stuff feels like fire if it gets in your eyes...I know from experience.

You could also use Marine paint.