If you care about seeing the water temperature of your water heater, and the distance between the Sonoff device and the water tank is less than 5 meters, check out the Sonoff TH Origin and the Sonoff temperature sensor.

If your water heater requires less than 7A, than the Sonoff BASICR2 is the best choice.

If your water heater requires between 8A and 17A, and you do not care about monitoring the temperature, than the Sonoff POW Elite is the best choice. As a bonus, you also get to monitor the power used by the water heater.