Thought: Most diesel engines, at least the ones used for stationary gensets, come with three grades of muffler, ordinary, residential, and hospital. Before you do major surgery on your boat, consider moving to a better muffler. Also consider how much of the noise you are finding unpleasant comes from the engine rather than the exhaust. You may need a sound resistant box for your engine.
Think of it this way. Cars have dry exhausts. Cars have diesel engines in the 100+ HP class. Cars have engineers who have dampened noisy harmonically vibrating fuel tanks
and body work
. Cars are not loud.