is a very complicated business, however there are materials that are designed to do the job. Soundown is one of the companies that specializes in sound insulation
. Yes it is expensive, but materials designed for the job do a much better job.
Airsep has equipment
to stop the splashing of the generator exhaust
and significantly quiets the exhaust
You need to think of sound absorbing materials as well as sound blocking measures and sound isolating (engine mounts, floating floors, etc.) in order to do an adequate job. Hiring a good engineer
with experience in these matters would probably be worthwhile.
Then you have to think of ventilation of the engine
Something you may be able to do is visit a high quality yacht of recent vintage and check out how they have insulated and ventilated the engine room. Pay particular attention to the isolation techniques they use as well.
This is not an easy task.