Step 1. Add 2 more speakers in the rear on the side.
So where are the other 2? Speakers behind you tend to do little for you, while underway. If there are speakers in proximity to the captains chair and they struggle to be heard underway, external amplification is needed. At the same time, I would still add that 2nd pair in the cabin.

Im a big fan of dual batteries in any boat. I consider adding this as well.