OK, understood, that's a completely different question.
There really is no "best" material, because, like boats, the sound deadening materials are a compromise of quality, cost, weight, and the space you have available (thickness).
Don't know how far you've gone in your research. If you've just started, then a Google search of "boat engine sound insulation" should give you a good start, and also look on Defender's and West Marine's websites. The Google search will undoubtedly show previous posts on this subject that may well include posts made on this very forum, so you can also search here, too.
That's what I'd do.
One of my dockmates did just that, then waited for a sale, the stuff, all of it, is pricey.
Good luck.