Yanmar YSB 8 circa 1980

[Halifax Sailor]

Have a Yanmar YSB8, works well, little noisy (go figure). Have a small leak, raw water somewhere when running engine. cannot find it, had the first mate drive, hung upside down in cockpit lockers until i was dizzy, not the shaft connection, can't find leak in exhaust line, doesn't seem to be the muffler box either. have attached diagram of engine. shows a cooling water drain cock. have never been able to move it from present position. have watched it, never seen it leak but afraid to put large wrench on it to make it move. Could this valve leak water when thermostat opens? The leak is definitively connected to engine running as i have nothing when sails are up. Looking for ideas?