Cant see this for the life of me! I would be amazed if bath water in waste could introduce earth potential, or any other potential for that matter. A 1m column of water with a high degree of disolved salts in a 15mm plastic pipe will have a resistance of circa 30,000 ohms! I can see however, that a poor neutral connection in neighbouring premises would increase potential between bonded bath and floor, which may be more representative of true earth. May be reasonable to sink spike to reduce touch voltages on pme systems where neutral problems will always exist.
