In a structure for mounting a sacrificial electrode in a cooling water passageway provided in a cylinder block of a water-cooled engine for an outboard engine unit, the cooling water passageway is defined by a groove portion provided in the cylinder block and a lid member closing an opening of the groove portion, a gasket member is provided between the groove portion and the lid member, and a screw fixedly fastens the sacrificial electrode to a bottom wall of the cooling water passageway. The gasket member has an extension portion opposed to a surface of the head of the screw located remote from the bottom wall of the cooling water passageway.