Probably a diode on the alternator has failed and the alternator needs to be repaired. Check with a handheld voltmeter, if you have access to one, from the power line post on the back of the alternator to a ground on the block to read the actual voltage. This will confirm that it is not the guage.
Operating the boat with the voltage too high will boil the battery and kill it over time. It may also damage electronics like your stereo or engine management system.