Elevated1,
The place to begin at this point is to maintain what you have and add what you lack. There are more linear in-boat speakers than the factory but once you add a subwoofer you will find the following changes.
* The bottom two octaves, once instated and properly powered, will add a great deal of fullness and perception of volume.
* When you convert your existing amplifier and coaxial speakers to highpass, narrow their bandwidth and responsibilities, the result will be much cleaner and substantially louder. Even plain speakers come to life.

Add a quality waterproof bass-reflex sub enclosure with a corresponding 10-inch driver plus a good monoblock amplifier.
You won't believe how dramatically different your system will sound with this easy and inexpensive upgrade. I would look at changing your in-boat speakers after acquiring your tower speakers in case you want to run a consistent cosmetic scheme.

David
Earmark Marine