Interlux Schooner Varnish is great.  I have also heard good things about Minwax Helmsman.

I think when they say not to be used below the waterline, they mean for boats that are left moored in the water, not trailer sailed or kayaks. Boats like vintage Chris-Crafts have bottom paint on them which allows them to stay in the water.  I use Epifanes, but any premium varnish would be fine.  

It seems like there is a huge information gap when it comes to varnish, and this seems like an awefuly big unkwown to this whole wooden boat building equation.

I have been unable to find any data on-line that gives a quantitative comparison of the relative stregnths of the UV protection of the various varnishes for sale.  Something like an SPF for sunscreens would be most welcome.

OK, now one of you chemists/ physicists out there tell us why this couldn't be done.

Hickory

I aint noany chemiophysiacist but I did use Epifanes high gloss extra uv on my Chesapeake 17 lt and it looks like glass.

When I Googled "Marine varnishes compared," I got the following:

http://www.practical-sailor.com/issues/37_9/features/exterior_wood_10576-1.html

Interesting to note that Minwax Spar Helmsman did well. Two part varnishes did best.

My 2 cents: I've used Epifanes but couldn't ever really become friends with it. Clearly a quality product, so perhaps it was my environment, etc. It always seemed too heavy and stayed soft despite mixing, etc.

Interlux Schooner: loved it. Easy to control. Great results. Reasonable price. Descent scratch resistance (none of them are stellar in this regard).

BTW: No part of a kayak qualifies as "under the waterline." That is referring to boats that live primarily in the water. I don't think there is a varnish for "under the waterline." So don't worry about that. Just be sure you are using a varnish with UV protection, which just about any boat or spar varnish will.

But, the truth is that varnish is more about technique than anything else. Any of the better brands will perform great if you learn how to apply them. Each seems to have it's own little qwerks (best temp, humidity, thinner, etc.)

Happy varnishing!