Please start by telling us how much electricity you tyhink you will use (multiply watts x hours). Also, how many engine hours per day (a 9.9 hp outboard typically charges 100 watts at full rpm, much less at lower rpm).
Also how much battery do you have (AH rating). Many small boats simply run on batteries overnight. If you are really talking about just a few overnights, a second battery and a modest 110v charger may be the best deal. I did that for many years on a Stiletto 27 (2 x group 24 batteries, 2 people), with trips up to 500 miles. We would plug in at a marina every 2-4 days. Simple boats take little power.
A short example:Sail Delmarva: Short Summer’s Cruise
Typically low outboard hours and rpm won't do much, and a 5w pannel will maintain but not charge a battery.