Conventional wisdom says that you can continuously charge a battery bank at 1/100 to 2/100 of its daily amphour capacity and not have to worry about a controller. So say you have a 200 amphour bank then you could charge at 2-4 amps and not have to worry about overcharging. This rule of thumb is somewhat related to the self discharge rate of lead acid batteries.

4 amps is roughly 50 watts. So you need a 50 watt panel, maybe smaller particularly if you have gel cell or agm batteries that self discharge at a lower rate. You can wire it to a ciggarete lighter plug and plug it into a convenient outlet when you are not on your boat. Use at least 14 guage wire and you won't need a fuse as the ciggarete lighter outlet should be on a 15 amp breaker which will protect the 14 guage wire.

Sorry for so much math. The bottom line is a 25-50 watt panel for a 200 amp hour bank. Larger or smaller depending on your battery size and type. For AGMs you can go much, much smaller as their self discharge rate is very low.

David