Many ideas
1. your battery was out of water
2. you haven't checked the charging voltage since you either a) bought the boat or b) installed the charger
3. your charger is NOT a TrueCharge 20 or 40
4. you may keep your boat plugged in all the time, although you did say you just plugged it in overnight; do you do this all the time?
5. you may not have checked the water levels in your banks regularly
If we're anywhere close here, you need to check your charger performance very carefully, check your water levels regularly, and stop plugging in your boat when you're not there. You most likely have a bad charger, or a good charger and a dry battery, and may have irrevocably damaged your battery.
Please let us know what kind of battery setup you have, how it's wired and what make charger you have, and the size of your banks. What is the voltage output of your charger?