I use spar varnish more than anything else. Varathane is pretty good, too, but seems to crack a bit, and water gets under the finish quicker. I think it may be because the spar penetrates better, or I do not apply the Varathane correctly.
Once water gets under the finish, freezing and thawing cycles do a number on it pretty quick, and it starts flaking off.
I know a professional stock maker who uses Super Glue, of all things, to finish walnut gunstocks. Says it is by far the toughest finish he has ever discovered. It is tricky to do, but is tough as nails and looks beautiful.
Ted