Supposed to be resistant to salt water and have more ultraviolet light filters, but as I can never get away with more than a season without re-varnishing it has never made any difference and some of the cheaper brands seem to last a bit longer.
I don't use polyurathane types externally anymore too hard to remove and they dont breathe. If its a small area have you tried clear epoxy and varnish on top. Works wonders Pig to remove when the time comes but my tiller was done 5 years ago and still looks great.
Iv'e had great sucess with blackfriers which is sold as 'yacht vanish' but through builders merchants and hense cheaper than many chandlers. Mainly I like the coulore and flow, but every wooden boat owner has a favorute varnish.
A very good comparison of their protective coatings.
Yacht varnish is usually one of the polyurethane based ones with additional UV protection.
Best were the two-pack varieties - however resistance to degradation is directly proportional to difficulty in application...
In addition to being UV resistant the marine type varnish is in theory more flexible with temperature changes and less likely to chip.
From the Pacific Coast,
Gary
Just a note of caution, I bought some 'yacht varnish', made by a brand leader, from the local DIY. In the small print it said 'not suitable for marine applications !!!