A shower is one of the necessary tasks that statistically consumes more water. Even so, there are different methodologies to make a better use, keeping the same results or even improved. Water is a solvent, one of the most effective ones, and the skin has a protection that gets weaker with intense water or excessive soap.
Starting from here, less water falling into your skin could be seen as positive. Tap water isn't precisely 100% clean as the government usually establishes several limits for contaminants, so below a degree, these impurities are nowadays considered safe in some locations. Other sources say that they cause different health problems, allergies principally. When you drink pesticides, radioactive isotopes, herbicides, plumb, organochlorine compounds, ..., perhaps something should happen including when they are found in little quantities.
In the shower, nothing obliges you to maintain a constant flow of water. Some heaters need a bit for raising the water temperature toward normal levels and cooling is of course not recommended. Anyway, that water should be stored in a recipient for later utilization. Why not for watering plants or cleaning the shower after use.
If you plan to wash your hair, when is wet, the flow of water should be stopped. That way is what you see in hairdresser's. Apply soap without water and rinse the hair after with it.
Same for the body; there's a considerable amount of time to apply soap and the tap doesn't wait for nobody. Not every water installation is the same, but say that is normal about 8, to 25 or more litters each minute. A lazy individual, 20 minutes dreaming about being in the Caribbean, potentially means 500 litters if his/her tap allows that flow.
From our experience, alternating the flow of water, or with other words, using water only when it's needed, the body gets accustomed to those slight variations in temperature. The heater also needs some time, although the water goes briefly hot other time.
When you take it as a habit, it doesn't require the minimum effort, and to withstand temperature changes is a feature! The only difference is analyzing consumption per year or decade; by billions.
Always the same word: responsibility.