Personally I prefer the phrase, we've all passed a lot of water since then, which means exactly the same thing if slightly less wholesome - it simply means a very long time ago. As far as the first example is concerned, it means, what's done is done and cannot be undone.

That's water under the bridge

and

There's been too much water under the bridge

I heard the second one over fifty years ago, but only once.

Do they mean the same or is there a difference?