The best thing to do is what is called "Navy Shower".
You get in, turn on the water and wet yourself down. Turn off the water. Soap and lather up.Turn the water back on and rinse off.
The tank will not fill up as fast, you save some hot water for the next person and you still "Showered".
I spent 8 years in the Navy and still try to figure out why we had to Conserve water when we were in the middle of an ocean and we made our water from the ocean water.