I have worked on hundreds of these leaking water pumps.  The mechanical (ceramic faced) seals seldom leak and are usually the first thing most arm chair LT mechanics replace.  The majority of the leaks occur between the bolt and the impeller.  If the impeller shaft protrudes above the impeller when the bolt is tight, the rubber faced sealing washer cannot form a liquid tight seal between the impeller face and the sealing surface on the bottom of the bolt that screws into the impeller shaft. 

When the shaft protrudes above the impeller, the impeller needs to be replaced and or the end of the shaft needs to be ground off so that the rubber faced washer can form a good seal between the impeller and the bolt.  The best repair is to have the impeller machined for a steel insert that prevents the water pump shaft from embedding itself into the impeller and allowing the shaft to protrude above the impeller surface where the rubber faced washer is suppose to seal.