Something's amiss - either you're leaving items turned on, or there is an excessive draw on a circuit. While you don't need to leave your battery switches "on", even if you did (but have other items turned off) it should not kill the battery. With everything on the boat turned off, do an amp draw test - you should have less then an amp.
As far as your charger goes, something's funky there, too. Either you're not actually leaving it on, it's not connected, or there's an issue with it. Test the charger.
However, all of this could also be explained by a bum battery. If this battery continually goes dead, and the charger doesn't charge it, how have you been charging it back up? Relying on only the engine once it's started? Pull the battery, take it home and charge it completely and correctly. Then get it load tested. How's the fluid level in the battery - when's the last time you added distilled water?
