I got a service call last week. It has turned into a mystery. The homeowner is getting shocks from the shower. Ceramic floor, basement bathroom, plastic floor drain, and 3- 8 volts reading. I took the service ground to the street side of the water meter. The voltage won't go away.
I had a situation a few months ago that I posted aboutsimilar to this. In my situation the customer got shocked whenever they touched the outside spigot with bare feet on the ground. It turned out the power company was "leaking" current into the ground from the Xfmr. All power in this area is underground, is your area on poles or underground?
Exact same scenario, had this happen, may be of some help to you.
POCO had loose (barely functional) neutral splice at pole, pole grounded properly - meaning a ground wire down to the earth.
Service grounded properly with a ground rod and water pipe ground.
Neutral path had such high resistance, the over current had established a path through the earth for a percentage of current.
POCO non-responsive, saying "It's still there" and would not remake splice, even though it was quite evident it was rusty, nasty, loose.
Climbed pole myself, cleaned and crimped new splice, problem solved.
IOW, check your neutral connections ALL the way back, I'm willing to be it is what you will find. If he has close neighbors, you can also be having a problem from one of them.
I'm reminded here of one Creighton wrote up where a solenoid from a neighbors washing machine was causing significant problems in the house because they were both tied to the water line, in my case the house was very rural so neighbors did not fit into the picture.
To solve the immediate problem: getting zapped in the shower, just bond both hot, cold, and the drain at the shower together. Assumedly all are metal, else how could the person in the shower be completing the circuit?
A neighbor had same problem about 10 years ago. Nobody could figure out why. Everything grounded properly, in fact, everything was grunded to everything else at multiple locations in an attempt to solve problem.
Turned out to be corroded ground rods at the POCO's substaion 1/4 mile from the house. Rural area, no other houses nearby to be affected.
Is the water heater gas or electric? I had a very interesting one where a subpanel was fed with 3 wires using the conduit for the equipment ground. Someone added circuits and landed the neutrals on the equipment grounding bus, thus sending neutral current through the conduit. Well the conduit eventually failed as a conductor and the neutral current found it's way back thru the furnace grounding conductor, which was on that same bus.The neutral current from the other loads in the panel was travelling on the furnace grounding conductor back to the furnace, where it then travelled along the gas line to the water heater, where it travelled to the water lines and out to the main. Whenever someone would take a shower, they became part of this return circuit. The gas pipe in the kitchen actually got so hot it blackened the ceiling where it passed thru to the attic, probably touching against the metal lath. Rewiring the panel and providing a 4 wire feed fixed everything, although at first it was not obvious to me why the pipes were hot and I thought there might be another problem. I knew the panel was wrong and fixed it right away, but it took me a lot of forehead wrinkling thinking in the days after to figure out exactly what was happening and why. Brian