This idiom is common in both social and professional environments. The idea of water moving on and not staying stuck in the same place. When something is considered ‘water under the bridge’, this mean it occurred in the past and has been dealt with, and is not worth dwelling on now. It’s generally used to refer to unpleasant or troublesome events that are in the past and consequently no longer regarded as important.
There was a time when I really didn't like Steven, but that's all water under the bridge now.
That supplier let us down with a shipment a while back, but they've since apologized and reimbursed us, so it's all water under the bridge.