2004 honda 150

[Buster1]

I have a 2004 honda 150. When running I get a intermediate beep with battery light on. Tach is not working,bate eris check good and in gear under load only have half power. I can unplug square plug on alternator and beep stops. Altenator is putting out 12.74 volts at alt.terminal.checked alt wire under dash and only getting 2 to 3 volts. This boat has only 4 hrs. On it . It has set for 13 yrs.being started once a year. Motor runs very smooth. Any help would be appreciated.