I'm a long-time woodworker, and built my own sailboat. I maintained the varnish fanatically, and also did some varnish brushwork for hire when I was cruising on her...
Modern spar varnishes and polyurethanes are remarkably durable, and almost all the quality finishes employ UV inhibitors.
If your finished surface will never be in direct sunlight, then the durability of polyurethane will make it the right choice. If exterior woodwork is to be finished, choose spar varnish.
Here's the reason for my bias: Clear finishes succumb eventually to ultraviolet light, much more so than through wear and tear. Eventually, even the most well-maintained clear finish will have to be stripped and taken to bare wood, (although this period may well be many years.) Polyurethanes are very difficult to strip, and could possibly require the use of chemical strippers... while good ol' spar varnish can be removed with a sharp scraper and some sanding. Either finish would need re-coating at the same time, anyhow.
One more point: Build up your coats with gloss finishes, sanding between each coat with 220 grit or finer. Then, for your final coat use the satin, eggshell, etc. for the look you want. This is because the gloss finishes are waterproof, while the dulled finishes are necessarily porous.
How many coats? Glad you asked! I can still hear the old British shipwright saying: "Apply seven coats. By seven coats, I do not mean six coats!" Did I always follow his advice? I'll never tell!