I added a powered subwoofer and a pAir of external small box speakers under the cabinets above the windshield.
This made a massive difference in sound quality. The active subwoofer is small and an option is the set of box speakers.
I ran the power back to house batteries using #10 wiring.
It has an auto on feature - i mounted the sub in the passenger foot well - the box speakers connect to outputs on the subwoofer.
The small box speakers are pointed back at the cabin and the sounds is really nice.
I had a local stereo shop install the wiring and the components. Total cost was around $1,000.
Here is the info:
Waterproof Radio and True Marine Entertainment Solutions | FUSION Entertainment