For what it is worth, here is how we save water while waiting for the hot water to arrive: Every night, DW is the first to shower. She takes a four-cup plastic measuring cup into the shower. She runs water from the hot water faucet into the measuring cup until the water coming from the faucet is warm enough.
The measuring cup usually contains about two cups by the time she decides the water is warm enough.
Then, she showers. (And yes, these are very serious Navy showers -- we mostly boondock.)
When she is ready for a final rinse, she adds a little hot water to the measuring cup and uses the warm water for rinsing.
Then it is my turn to shower. The water in the hose is already warm, so I don't need to use a measuring cup.
Someone mentioned Oxygenics. We got one for the trailer and found that it uses a little too much water -- about 3 quarts per minute, on a minimum setting. We found that a cheap little low flow showerhead consumes two quarts per minute on a minimum setting.
The Oxygenics is now in our S&B house -- it's great.