From their website, it seems that Hydro1 is only "saving" because it is using a smaller 1.5KW heater. If using a small capacity heater, it will need to run a longer time to heat up the water. In the end the power consumption will be the same.

For example, if using 3kW, you'll run 1 hr. But if using 1.5kW, you'll need to run 2 hr. The end result is still the same, 3kWh.

There's also some water heater that says it is using DC heating element to heat up the water and also energy saving. But it doesn't save more compared to conventional electric heater. AC or DC, the power consumption will be the same regardless of the voltage as the heating energy required to heat water is fixed.

For information, to heat up 30 litres water by 25degC, will need about 0.87 kWh of heat. As long as it is heating element type, it will need to use 0.87 kWh power input.