Here is all you need to know about the varnishes, wood finishes and aliphatic Polyurethanes on the market suitable for marine coatings.
The take away point I think is that whilst these test pieces have been left to deal with the vagaries of nature and a couple look like will go 10 years without “failing”, in reality, for all but Coelan and Uroxys (Awlfgrip) you cant go more than 2 years without applying a maintenance coat. All the products ket their integrity past 2 years without any maintenance, but they pretty much all experienced some sort of failure on arrises around 2 years, maybe 4 at most. In the marine world, you can’t really let that go for any length of time so apart from the PU’s, you would have your brushes out every 2 years for preventative maintenance.
Another thought is that many decisions in the wooden boat world hinge on aesthetics. So you have the option of long lasting Sikkens Cetol which I have seen used a lot on rails and detailing, but that product would not be seen dead on a classic boat where the owner wants high gloss varnish brightwork. So then you are back to choosing between varnishes that may go 2-3 years before you are on the maintenance cycle again. Or looking at Coelan which aesthetically is as “dodgy” as Cetol.
Hope that link helps though, it is a very interesting read, I think