I varnished over paint once....and will never do it again!
Unlike auto clear-coats, traditional varnishes (including Captains) are golden in color and they darken substantially as they age. Within a couple of years you will start to see just how unevenly you applied it as the color starts to change and get blotchy. Mine was sprayed, which is about as evenly distributed as I can get it and even so, it was bad enough that I had to repaint over it.
I don't know what the abrasion resistance difference is between varnish and enamel, but the UV resistance is substantial and quite different. Paint will oxidize on the surface. If it starts to look chalky, you can polish it out with polishing compound, or a cleaner/wax like 3M "One Step"and keep doing this until it gets so thin that you start to be able to see through it. We had an old Hobie Cat that I repainted which then sat on the beach uncovered for nine years. It was getting pretty nasty looking, but we were still able to polish it up to a respectable state when we finally decided to sell it.
On the other hand, varnish, being clear deteriorates all the way through as UV starts to break it down. You have a choice of sanding the dead varnish off and starting over, or periodically revarnishing to add a fresh layer of UV absorbers to protect it. This buildup will tend to increase the blotchy look as the various layers darken. Picture your nice paint job with five or six layers of varnish, some of them weathered and dark over it - because that's what you will eventually have.
In short, there is a very good reason why none of the manufacturers of marine enamel suggest top-coating it with varnish. They're in the business of trying to make paint that goes on well and lasts as long as possible with as little maintenance as possible. If varnishing over it was a good idea to make it last longer it would say so right on the can.
