Lead sheeting works the best for soundproofing. Foam (on the outside) also works really well.
In an 80' steel commercial boat with blown in foam (sealed in with fiberglass) with the main engine and 3 other diesels running, while in the crew area, I could not hear the engines at all. The only indication of machinery was a slight vibration in the galley table. Engine room was directly below the galley.
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