From your description, I think you have good reason to be scared.
It does sound as though someone has just tried to ground the metal case of the shower to the neutral. This is VERY dangerous, because if the neutral wire ever became broken somewhere along its length, one of two things would happen:
1. If the shower is connected with metallic water pipes and these are properly bonded to ground, then the current that normally flows in the neutral conductor will flow along the pipes instead.
2. If the shower is coupled to the water supply with plastic pipes, or with metal pipes which are not properly grounded, then the whole metal casing of the shower would become live. In fact, if metal pipes are used but are not grounded correctly, this could also energize every other water fitting in the building.
I would suggest that you have this checked out immediately.
By the way, do you know the power rating (kilowatts) of the shower? These instantaneous showers have become very popular here in England, but even the smallest older units were 6kW. Such a unit designed for 127 volts would need about 47 amps of current, so the cable feeding it would need to be very heavy.
