Like Gnoahhh I have spent a lot of time trying to keep up with the brightwork on boats. I have built a few, eaking out a living in boatyards in my '20's and have long owned a 35' cutter which I built in the late '70's. Teak decks, hatches, rubrails, bulwarks, etc, etc. require a huge amount of effort to maintain here in the northwest and it is even worse in the sunbelt.
A clear epoxy base followed by a dozen or so coats of a high quality spar varnish holds up well - far better than varnish alone, especially if the varnish is followed by a half dozen topcoats of a two part clear urethane such as Awl Grip (it is a bitch to remove though and I have moved away from this technique). I don't have a favorite - there are several really good brands although I believe the Epifanes may well be the best I have personally tried. I am a believer in thinning the first coat finding it does appear to improve adhesion - YMMV. Thinning the last couple coats will allow a smoother finish.
In years past some epoxy coatings fogged in the sun, ruining the finish. I have not seen this in recent years and there are several epoxy resins now built with this application in mind.
The absolute best final coat is a full boat cover (with a great deal of ventilation built in).
