Yes, it is called an Atmor, and it is carried by some hardware stores. The serious plumbing suppliers wont touch them. My info is that they are too small a heating element in a very tight heat exchanger and they succumb to avnit (scale corrosion) in no time. In areas with soft water,( NOT HERE!) it is a legitimate product called a point of use (POU) water heater. If you have enough watts going into it, and dedicated piping, it will give a fixed flow rate enough for a shower.
twisted,plumber in Jerusalem