Labattsman

First run of the year and my voltage gauge was showing 11 volts. Verified low voltage at gauge with a meter, been working on learning somewhat how the charging system works in a twin boat, ran engines individually and both are charging the house battery fine. Pulled batteries back out to have tested at auto store and both test fine. Made sure all connections are tight and verified grounds between engine block to battery and wall ground plate. If I jumper the batteries from house to engine battery it charges engine battery fine @13.6-13.8 volts. What the hell am I missing here??? Also what is the purpose of this in picture ?