How to kill the Watcher in the Water

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? Did it suffer mass depression like the rest of the creatures of Sauron when the ring was unmade? Was it just left there with everyone entereing from the other side? Was it killed somehow?

If it was killed, then how? Several options present themselves to me;
1; hack and slash - a large force came and just chopped it up into little peices.
2; starvation - keep food away and the problem will go away.
3; breach the dam and then do something.

1 has inherent problems that it would take a lot of effor with potential for significant losses and the watcher could just retreat into the water and not play if threatened.
2 the issue of food never was that important (save for Hobbits deprived of one of their many daily meals). Perhaps even the watcher fed on aquatic life and kept a breeding stock alive in the lake.
3 with blasting powder as used in the assault on the Pelanor to breach the wall the forces of Sauron used blasting powder (as did the Uruks in the Two Towers movie), so this could be used, or just brute force and engineering to breach the dam, then as the water level receded options 1 and 2 above could become more viable. The watcher may also require water to breath or live in or something and may well die without it.

Any thoughts? Alternatives? Falicies in the above rambling?