These days there's heat recovery drains. The idea is generally to first make sure you use a thermostatic tap for the shower and then use the drain water to heat

These days there's heat recovery drains. The idea is generally to first make sure you use a thermostatic tap for the shower and then use the drain water to heat the cold water line going to the tap. The thermostatic tap will adjust to keep water temperature steady as the incoming "cold" water gets warmer over time. The marketing material claims energy savings of up to 50%, but presumably you'll only reach that amount of savings if you take hour long showers at ridiculous temperatures.