When you are down at Princess Auto pick up a load tester $40. Every boater should have one.
As stated every battery should have 12 volts any thing less and the battery is done. Fully charge battery then un plug note voltage then flip switch for load test, Plug charger back in and test battery for voltage, the voltage should go up, if it didn't it's the charger.
You can also check if your alternator is working by starting your main engine same procedure.