Is it normal to have voltage between shower faucet and drain?
I measured 2 volts between shower faucet and drain when water heater is off, and 11-12 volts when it is on?
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No, this is not normal and dangerous. I suggest that you hire an electrician or least favorable option (ground is like crazy science to regular person and there are a lot of gotcha ya), do your own research. It looks like there is insufficient grounding and your heater or some other appliance has a ground fault. All of the electrical circuits and appliances must be grounded and all metallic piping must be bonded.
Damaged skin reduces your body’s natural resistance so you felt the shock.
This low voltage can be generated in many ways and at that low of a voltage is not dangerous.
the National electric code article 411 low voltage lighting: think swimming pools hot tubs where this lighting is normally used allows 15 volts ac where wet contact is likely.
I regularly bond the hot water pipe to the cold water pipe if metallic many jurisdictions require this as the NEC if likely to become energized it shall be bonded to the grounding electrode system.
Another possibility is an imbalanced electrical system. A heavy imbalance can cause a voltage to be present on the neutral & grounding system that a well balanced system will not have.
No one likes getting shocked so taking steps to eliminate the cause (other than broken skin) like making sure the water piping is grounded both hot and cold. Getting the home balanced L1 to L2 I have seen 50 amp imbalances cause much higher neutral voltages and once the system was balanced no voltage on the grounded or grounding conductors (moving breakers to even out loading for each leg.)
The good thing is this is not considered hazardous but I would be at least adding additional grounding to eliminate it.