A good , fully charged, battery should read about 12.7 to 12.8 volts when it has rested for about 12 hours with no load applied after being charged.
A half charged battery will read around 12.2. Below 11.8 the battery is to all intents and purposes fully discharged.
When you crank the engine the volts will fall, as low as 10 volts perhaps, but recover when the engine is running. Once you are running the boat and the engine is operating in its normal rpm range the volts should steadily rise as the battery recharges. With a regulator rectifier , like your engine has, the volts should steady out at about 14 volts or a little more.
Check these readings with a known good digital voltmeter or multimeter connected directly to the battery terminal posts.
I would also attempt to check the boat voltmeter by connecting my multimeter across its terminals.
If you find the boat meter is reading correctly and you don't get battery voltage readings rise as above then you probably still have a charging problem.
If your voltage readings start off below 12.7 then either your battery is not properly charged or, if you cannot charge it up with a battery charger to this level, the battery is no good..
There is a good section in the CDI electronics trouble shooting guide on battery and charging circuit testing
See
http://www.cdielectronics.com/troubleshootingguide/
Never allow a battery to remain in a discharged or partly discharged state for longer than absolutely necessary.
If you connect the battery with the polarity reversed, even for an instant, you will toast your rectifier.
If you disconnect the battery while the engine is running you may damage the rectifier.
