On this or another list 3-4 yrs back there was an excellent discussion
of this topic with one very good extended post. A google of the web (vs
groups) "boat engine noise sound-proof" would include the archives of
several private groups. Sorry, don't recall the source.

Basically:
- block/caulk all air leaks out of noisy area; note connecting bilge
areas, spaces behind ceilings, communicating lockers, utility
penetrations, utility conduits, cracks/spaces at edges of non-structural
bulkheads, etc. All hatches into the noise area need to be tightly
gasketed. This leads to providing combustion air and cooling ventilation
to the engine space from outside.

-stiffen any large surfaces so they resonate less

- install sound deadening on all surfaces of the engine space; there are
material specs to check, but you'll find those somewhere

- isolate the motor and the driven appliance from the boat to prevent
sound and vibration from getting transmitted directly to boat structure;
note hose attachments, fuel lines etc. Nothing should touch anything
(skyhook engine mounts... g)

- keep the mechanicals tuned to reduce vibration

There are clearly compromises involved. The discussion covered some of
these and would be worth looking for.

Rufus