How to kill the Watcher in the Water

[gorthaur_cruel]

I have been thinking, presumably Moria was re-taken so Gimli could obtain Mithril to remake the doors of Gondor, so what happened to the Watcher in the Water?

I doubt Moria was retaken. There was still mithril around in Middle Earth, it was just really rare. Tolkien once wrote an epilogue to the Lord of the Rings, which was never published but can be found in HoME. There, it says that Moria was not retaken.