Catch the cold water that comes out of the faucet, tap, or shower while you are waiting for the hot water. Use it to water plants or pour into your toilet reservoir after flushing.
Water from a hot water tank may have more sediment or rust than water from the cold water tank, but is otherwise suitable for drinking. If you use a water filter, you can filter the saved water, and put it in bottles in the refrigerator for drinking water.