I might have hesitated on the polyureathane recommendation, but then again, I don't know everything. I don't know what the exposure to that much moisture is going to do to it.
Around here I have seen either marine epoxy/paint or my personal recommendation; spar varnish.
I use spar urethane for topcoats. I figure since it's good for outside, it should be good for a bit of saltwater splash. Check to see if the poly you want to use is for inside or outside. That's why I went with spar urethane.
Inside my stand is just white oil based enamel. Flat white that I had from inside my canopy.
Spar varnish is fine, the only advantage it has over Poly is that it is more resistant to the suns uv rays. Which might not be a bad thing thing in a reef tank. The poly is much more durable on interior surfaces, it is eaiser to work with and drys faster. I do not have any varnish or stain on the inside of my canopies. I use stain and poly on the outside.
You can also use an acrylic polrureathane that is nontoxic and will dry in seconds.
Andy