Just a side note ....
In many ways, this arrangement is the norm in many "modern" parts of the world. Let me explain ......
First off, having the water heater literally hanging in the shower, as shown, is typical.
Having a switch is also typical .... that is what controls the water heater. That is, if left on, the water heater will ultimately start venting; there is no thermostat. You actually turn the thing on, wait a bit, then take your shower - turning it off as you exit.
However, the switch is more commonly mounted on the outside wall of the room, and often has a pilot light, letting you know its' on.
Having the switch right next to the heater is a new twist.
As is the receptacle; typically ordinary flex is used (not sealtight).
One detail not obvious in the picture is that the entire service is likely RCD (GFI) protected.