I disagree. If there is current flow on the water pipes, there is a ground fault somewhere which needs to be located and corrected. Also, your water heater probably already has a metal connection between the two pipes. Over 99 percent of water systems have no wire bond between the hot and cold pipes, and it has no effect on the longevity of the water heaters. Edit to add: I have been in 100's of homes and businesses doing electrical work, many service upgrades and what I am saying is from decades of experience.