Figures of speech are interesting. Sometimes they illuminate, sometimes they amuse, and sometimes they lead us astray. Far far astray – up the creek, down the steep mountainside, into the brambles.
If you do a bad thing, and you never take it back or apologize for it, and it does spreading harm for a period of years…and then someone asks you about it, the correct answer is not “that’s water under the bridge.” It’s not water under the bridge. There are reasons for that.
“Water under the bridge” is the right expression for something that’s over. It’s not the right expression for something that did damage that was never repaired; it’s even less the right expression for something that did damage that was never repaired and that is still happening. Like, if you make and sell a toxic food product, and that product is still on the market and still toxic, and someone asks you about it, you don’t get to say it’s water under the bridge.
The mere fact that something started a long time ago does not mean it’s now water under the bridge. The mere fact that you, the initiator of the something, are tired of it, does not mean that it’s water under the bridge.
Now if you have a quarrel with a friend, and then the two of you talk and it gets resolved, that becomes water under the bridge. If a third party asks about it you can say it’s water under the bridge. That’s fine. It’s resolved, it’s over, it’s in the past. But if the step with talking and resolving never happened, then it’s not water under the god damn bridge. It’s still dirty smelly water right there in everyone’s face.