The choice is limited. Halyard Marine sell a range, and I have always gone for the thickest and best I could afford. We also have an 'Aquadrive' to insulate the engine from the boat hull. As a result our engine is very very quiet.
By the way, there is a compromise between sealing the compartment to not allow noise out, and allowing air in somewhere to permit the engine to 'breathe'. Diesel engines need lots of air...
This is the stuf you need. Use the Maritex 32mm. It was enough for our boat to pass MCA coding reguirements for fire protection. Sound insulation is outstanding. Not cheap and it's heavy (a box of 4 sheets weighs in at about 24kgs) but very very good.
asap supplies do a similar product with the lead sheet barrier layer which is cheaper. I used it recently. Happy with it so far. it needs screwing in place however. I wouldnt rely on glue alone as the foam lead joining can seperate.
Also fitted Halyard last season, it's now quieter in the cabin than the cockpit, which means I need to fit some in a box around the after end of the engine, under the cockpit, just like the nice man from Halyard told me to at LIBS. For sound, every orifice is important so don't forget to fit a bit under the sole, i.e. in the bilges (if configured like my boat). Can't speak for the fireproof qualities and I hope I never do find out from experience.
Having rad many descriptions of engine insulation I found that enclosing the engine with 2 layers of thick plywpood lined with a stainless steel sheet provide a great deal of sound insulation and minimal fire risk.
We are enclosing the Yanmar with 2 sheets of ply (16mm X 2) and then the stainless steel fireproofing augmenmted with the auto extinguisher FM200.
It appears that the polyurethane engine insulation often covered in silver mylar or similar, does in fact burn with cynaide released.
Funny how that is omitted in the ads!
In the US there is paint sold that appears to deaden sound and believe that it is made from glass beads and latex water based paint.
This is similar and without the chandlery price tag....