1. John and I were married and divorced several years ago. I don’t often think of him or wonder where he is now. That part of my life is water under the bridge.
2. Mary Ann had a bad experience when she was young, but she doesn’t let herself think too much about it. It’s water under the bridge.

The expression makes the analogy of life as a river of water. The water that has passed under the bridge is that part of a person’s life that is past.