No poly resin is waterproof. Thats why they paint the parts of boats that you can not see, parts that no one will ever see unless you cut the boat up. Poly resin absorbs water. There is no such thing as marine poly resin. Poly resin has to be covered with paint, gelcoat, etc.

I would go with epoxy as it is easier to repair in the future. Only epoxies will bond to cured Poly Ester.

I have used West, but I like MAS as it mix 2:1 rather than some odd ratio. They have several viscosity resins and the hardeners come in 3 different cure speeds. There are others on Ebay with tutorials. MAS has videos on their site.