There's nothing about an RCD which limits the magnitude of a fault current (whether through a human being or anything else) - that is simply determined by Ohm's Law. All a 100mA RCD will achieve is to limit the duration of any fault/shock current of 100mA or more to an extent that (at least with a 30mA RCD) considerably reduces the risk of serious injury or death. However, with a 100mA RCD, teh degree of protection is much less, hence the risk of injury/death much more; 100 mA RCDs are not designed to protect human beings.
We should probably also add the usual caveat, that if one touches the live and neutral conductors, but is not in contact with earth, an RCD offers no protection at all.
An electric shock requires two ( or more ) points of contact on the body. So one is the water heater, what is the second one ?
If the second one is a metal simk of similar then fault may be at the sink and not at the water heater.
Use a true ground reference, the ground rod if it known to be good, and measure the voltage on the water heater where the shock is being felt. If it is below 50 volts then suspect the second point of contact and measure the voltage on that. But also investigate that 50 volts as that is not good.
It could be the floor is live due to a damaged cable and the water heater is at true ground potential ( Thermal insulation under floor can also insulate the floor from ground )
As this could literally be a matter of life and death the only place to start is to get someone in who knows how to find and fix the fault, and to get them in as a matter of great urgency.
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