So a huge reason showers, especially long showers, use a lot of water is that the water is only used once before it goes down the drain, but what if we had the option to reuse the shower water for each session? Fill an internal reservoir or the tub with just a few litres of water, and use a pump to recirculate it, with electronic controls for when to drain the water. It could be like a dishwasher: use one round of water for the main washing that actually gets the grime off, and then a little bit more fresh water to rinse completely clean. Something like this could also allow you to take as long a shower as you want or have one of those massaging showers with a rain head and like ten body sprays, and still use less water than with a conventional shower. A panel would control when the water is replaced and when the recirculation is disabled (for example, when using soap so soapy water doesn’t spray back into your eyes). I think something like this would go a long way toward reducing household water use.