I want a quite boat.  It is one of the things I am picky about.  I can't stand being inside a boat an unable to talk to people, listen to music etc... when the engine in on.  So I am going the extra mile to make sure my boat is quiet.  

Step 1.  Add 1 inch Safe and Sound Roxul insulation.  It is a stone fiber insulation that does NOT burn.  
Step 2.  Add 1/2 inch plywood.
Step 3.  Add 1/2 inch foam insulation with the high density rubber layer and a shinny surface.  Shinny surface doesn't do anything for sound, but will help reflect light and keep the engine room bright.

It is still a work in progress.  I am waiting to do a lot of things in the engine room for my exhaust system.  I have some pipes getting put together and once those go in, I can attache the fuel line, exhaust lines, risers, etc.. etc...  This insulation will eventually completely surround the room.  Haven't tested it yet.  I have a access to physics equipment, such as decibel meters and will eventually let you know how it works.