Water heater ground

Question

The water heater was installed (1967 home) with only two wires (240v) and no ground wire. I presume that it would have been meant to ground through the original pipes, however at a later date pvc pipe was installed on cold and hot side of the heater. Would it be safe to place a grounding strap from the water heater to the metal pipe on the other side of the pvc splice, or is it necessary to run a ground wire all the way back to the breaker box? Which is safer?