bc, there are 2 kinds of water resistance involved; liquid water and water vapor. Polyurethane ranks high against both. Film thickness is an important factor and the more coats, the better.
Spar varnish is not as good a choice as it would seem for this application. The main difference between spar varnish and other oil based varnishes is the UV protection. This is important for the outside of your wood door or you 50 foot yacht but it will not do much for you in the bathroom. Go with oil based poly.
I tried Formby's years ago and was not real impressed with their "system" personally. It is really based on the reamalgamation of a pieces present finish. This gets some of the dirt out of the finish but not all of it. And I strongly disagree with their use of steel wool for removing finish. I have seen the red/brown spots come out of a finish because of the little strands of steel wool that got stuck in the surface and then rusted after the finish was applied. But then I am old and crotchety and like doing things MY WAY!!! LOL I prefer using Citristrip and getting all the old dirt and finish off and starting over.
But it does sound like Formby's cotton ball test would work.