I have some speculation on a different track, make of it what you will.
The way that the dwarves referred to the creature as "the Watcher in the Water" always made me think that the pool had been there before Balin's company arrived, and they found the creature there before them.
The word "Watcher" is what grabs my attention. It makes me think of something that was there already, waiting for something or someone to come by for dinner.
Also, if the Balin's Dwarves had created the pool themselves as a defense mechanism and then the Watcher came, I'm not sure that "Watcher" is the word that they would use to describe it. "Intruder" might be the word that they would use.
Of course, I suppose that it is possible that this was a creature that the Dwarves had prior experience with back in their glory days in Khazad-dum, and it picked up the name back then.
So, the pool might have been the result of an earlier natural event and the Watcher came up after that, or maybe the pool was the work of the Watcher itself.
Anyway, that might be poorly reasoned speculations as a result of the connotation of the word "Watcher," but I thought that I would throw it out there for discussion since it presented an alternative theory.
(Although this theory does ignore the possibility that the Watcher is the Balrog, so I guess I don't get a bone.

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