This is and always will be an interesting topic. I looked at this years ago. My boat came with a good amount of sound suppression in the engine room, however on long trips the engine noise does get to us. As explained to me, because the vibration from the engines causes the entire boat to be a sounding board (like a guitar body), suppressing the sound is not all about the engine room. Ceiling cloth, curtains, carpet underlying, etc. all help. Of course so do good engine mounts.
I own a music school in Brooklyn and we came up with the same problem in our drum practice rooms. Just impossible to stop the vibration causing the duct work to transmit the sound to the tenants above us (caused a few complaints to the landlord). We already had 2 feet of foam installed above the ceiling when we built the rooms. After some research, I found the best option was "sound absorption", not sound proofing or suppression. We ending up buying specially made floor to ceiling sound absorbing mats.
In the drum room we actually doubled them. This caused an absorbing affect and it worked. Not soundproof, but deadened the sound by absorption.
With the above in mind, I would suggest following the same mindset and use sound absorbing materials both in and around the engine room.