Tankless Electric water heater Electrical interference
From the Rheem website for this model: 


  "Most advanced self-modulation, adjust power to meet hot water
  demand."


Most likely, that means it is using an SCR based controller (solid state power controller) to do variable power control of the heater elements and the phase-angle firing of the SCRs is causing extreme line noise and harmonics that is affecting other devices when it is on. Not a lot you can do about it, but in small doses as a HW heater would run, it's not particularly harmful. If it is causing severe negative effects that you can't stand, you could try having someone install a Line Reactor or an Isolation Transformer ahead of the HW heater. That will not be inexpensive though.