The article does put some numbers to the energy needed to heat the water. They estimate it at being equal to driving 3.5 – 7 km with a relatively fuel efficient

The article does put some numbers to the energy needed to heat the water. They estimate it at being equal to driving 3.5 – 7 km with a relatively fuel efficient car per shower. They also say they didn't count the energy costs from water treatment and distribution. And they also point out that all of the current wind turbines in the world could only power 1 billion people's daily shower, only considering the heating of the water... If there's a way to reduce the cost of showering without any significant decrease in quality of life, why not encourage it?