Are you willing to attach the panel(s) and charge controller to the boat? If so, can you dock the boat with the panel(s) pointed at the sun? If you can say yes to all that, I would go with an 80-100 watt panel and a Midnite Brat PWM charge controller and enjoy. Your math is as follows: 50% usage on the battery, 42 AH. To replace those AHs, you need to add them back and account for the internal resistance. You will see three amps replaced continuous during peek solar production and account for the internal resistance, that means you need 14 hours of peek charging. Since you don't have peek charging all day long, you charging will take around two days. Add cloudy conditions and you will need longer recovery times. Hit your batteries harder, and you will not be able to recover with less than a 120 watt setup.
