As mentioned twice, tight fitting hatches and sealed wire and plumbing penetrations can account for a lot of noise, and are the least expensive areas to address.
The foams used for noise insulation are open celled, to absorb noise, but also absorb the nasty atmosphere in an engine room. These contaminants are what cause the foam to break down prematurely. The silver facing is a vapor barrier, and it is recommended to tape all edges and seams to seal the foam when installing.
Some suppliers use premium components, cost more and last longer. On the Pacific rim it's buyer beware.
Mass stops noise, if what you are installing isn't heavy, it's just not going to work very well.
The sound "tiles" many Asian manufacturers used will absorb some noise, and thereby reduce a little noise in the engine space and also throughout the vessel. But to stop noise you need mass, like 20 years ago it was lead, today its mass loaded vinyl. These mass layers work best when "floating" between a de-coupler layer against the bulkhead/overhead and an absorber layer towards the engine.
For those with carpeting an acoustic carpet underlayment is easy to install and very effective.
And as Datenight said, an isolation product (an engineered foam) between the framing and the floor will decouple the floor and make a boat much quieter. Combined with good engine room insulation and a floor treatment like a carpet underlayment this is the best noise control available for boats like trawlers.
The spray/trowel liquid damping products work to various degrees for their intended purpose, which is damping structure-borne noise. Most also call for several coats, up to 4 or 5, to be effective. Most of the issues on trawlers is airborne noise, and a structure borne solution just doesn't work well for an airborne problem.