Once upon a time I had a sailboat with an SR2. The engine was lag bolted to wooden bearers, had the then usual non-flexible coupling that required a lot of shimming to align gearbox output and prop shaft. Had the old style stuffing box.
When running, inside the boat, it sounded like a destroyer running at war emergency power, but the vibration was not excessive.
I would look for some of the suggested causes, but would bet it is not the basic engine design. There things were a common life boat engine, and were reliable and trouble free.
Cheers,
Hugh