That stuff does very little for soundproofing, it is basically silver bubble wrap. (ie, a layer of mylar, some air, and another layer of mylar) Essentially it is more or less just air that is contained in a stationary pocket. Works great for temperature insulation though.
Didn't check if iboats has it, but search for soundown marine. That is marine rated sound proofing material that will do exactly what you need. To do soundproofing effectively, you need both heavy stuff and light stuff. Heavy stuff works by mass dampening and generally cuts out low frequencies. Light stuff (foam) absorbs high frequency sounds that have small wavelengths.
(used to build lots of car audio competition vehicles, it was standard to do a layer (or multiple) of the heavy stuff, usually raammat, followed by at least one layer of foam on top of it, usually ensolite.)