This is considered a barrier and deadening material for sound. I've done extensive research on sound deadening, since bringing the sound level down in my diesel truck. (I've taken it from 99 dB down to 82dB) - that's a *huge* decrease, as the scale is logarithmic, and I'm not done. Boat will be soon to follow, I promise.
The largest problem with the boat is twofold, the barrier and deadening layers are good - the deadening layer won't do a heck of a lot for us in the boat department though, due to the lack of resonance in fiberglass, whereas in sheetmetal it is much more prone to resonate. It will help, but not greatly.
What you really need is an absorption layer in addition to the barrier, deadening, and decoupling layer. The problem is, in our boats, this is going to be a significant challenge to do without significant modifications.
MLV - mass loaded vinyl is really the missing link in the equation here to block sound, and that's what you need here - something to completely block the sound. MLV is very heavy, about 1/4" thick, and very hard to "adhere". I might experiment with some on my current engine hatch. If you could get a layer of MLV to adhere, then add a layer of Ensolite closed cell foam on top of that, that would be a relatively thin, and very effective sound absorption solution.
Here's some good reading on the actual science behind what's going on.
I actually have some of their larger engine box insulation in stock, but there isn't room inside my engine box to place it, at least on the sides, there may be enough height to put a layer on the top.
All I can say is stay tuned - probably this coming fall and winter, and I can tell you a bit more about what I've found to be effective.
I've got thousands of dollars and quite a few hours in the truck project, and it's completely worth it... I will apply what I've learned there to the boat.