I've run across the same problem at times and what 69B-Cuda says is good advice. Even putting sealant on the threads sometimes fails to stop the leak. I've had a couple of chronic leakers that needed more. What I did was to use a tight fitting washer under the bolt head and then add a rubber O-ring under the washer. It was a little trial and error to get the correct size that would fit tight into the water pump bolt hole and tight around the bolt shank but it ended up working great.
Good luck, Mark