The characteristics of a heater like that, or an incandescent light bulb, are kind of funky. The device is a resistor, but the temperature changes a lot through each 50Hz or 60Hz mains cycle. This leads to substantial resistance changes through each mains cycle. The result is considerable THD. If you apply a nice pure mains sine wave and measure the current waveform you may see 10% to 15% THD. If you try the same thing with a typical water heater you will see very little THD. The much more effective cooling in a typical water heater results in much smaller temperature cycles, and the load is close to a simple resistor with a hint of inductance, and a smidgeon of capacitance to ground.