John,
I have not had good luck with that particular product, specifically regards water. I have a counter top in my entryway that water sat on and penetrated the Helmsman. Didn't raise it-just discolored it. Of course I blamed it on my wife as she applied the three coats!
I think the name implies something the product does not deliver.
I would recommend Varethane. It's pricey and not as easy to find, but really holds up.
One other technique I've heard of, but not tried yet, is to coat the wood with west system epoxy and then apply whatever synthetic varnish you wish. The epoxy does the wood protection part and the varnish takes care of the "look". I'd probably experiment with stain, epoxy, varnish on scrap first to see what the end product will look like. Of course the epoxy alone would be fine for unexposed to sight portions. Given that you are using plywood I would soak in a couple of coats to the end grain as that is the most susceptible to moisture penetration. When you drill your pilot holes for your screws you can "butter" each of the screws with epoxy and thereby seal those holes as well.
Good idea on the lighter colored oak.