Really electricity is an unseen killer.
Tingles sound trivial, but in a shower, not.
You may or may not have a main earth and ground rod, but your electric water heater should have a good (as in you don't want to die) earth. Both of them.
Get an electrician to check the water heaters.
Do you have an RCBO or earth leakage circuit breaker for the power to the water heaters?