Leave the stereo as is and buy a fresh air exhaust, makes the boat supper quiet, no fumes, plus you can actually talk to the surfer or guests in your boat. No need to make the radio louder... :)
Having said that I recently replaced my speakers with Polk Audio DB651 6.5-Inch Coaxial Speakers and Kenwood KAC-8406 4-Channel Car Amplifier and a new head unit for around $500. You can buy the fresh air exhaust and the stereo for $1000.. :)