Not all sailboat engine panels have an oil pressure gauge. In this absence, to determine whether an engine-running low oil pressure warning light & horn is a genuine or a false warning, remove the sending unit and connect a continuity device between the wire terminal and the metallic body of the sender. Continuity should exist. Then, with a dinghy air pump or bicycle pump suitably attached to the sender's threaded fitting, apply some air pressure into the sender (simulates oil pressure). If the continuity remains uninterrupted, the sending unit is faulty and can be replaced usually for under ten dollars. Your local marina's mechanic will do the same thing but at the going hourly rate that you're painfully familiar with. If the unit's electrical continuity is broken whenever you apply air pressure into it, the sending unit is OK and is truly warning you of an impending serious situation.