There are various phrases that people use as synonyms for "water under the bridge". One of the common phrases is "old news" which refers to an event or situation that has passed and no longer matters or affects the present. Another phrase is "ancient history", which means something that happened a long time ago and is no longer significant. "Yesterday's news" is another synonym and is used to describe something that is no longer interesting or relevant. People also use "bygones" to refer to past events that are now over and not worth discussing. In essence, these synonyms help communicate the idea of leaving the past behind and moving forward.