Does the boat have a voltmeter on the instrument panel? If so, what does it read engine off/key on and again with engine running at 1500 - 2000 rpm. Presence of a voltmeter says there is a charging system. What the voltmeter reads tells you whether or not the system is working. Static voltage on a charged battery will measure about 12.6 volts. That same battery measurement on a running engine should read about 13.5 - 14.5 volts. If the reading stays the same in both conditions and is less than 12.6 volts and drops below that when accessories are turned on, then the charging system is not working.
