Hello. This is an interesting topic, which most readers of LOTR probably have thought about in passing. I actually thought PJ's version of the Watcher was acceptable. The concensus seems to be the Watcher was one creature (for example, the somewhat dated
complete guide to middle-earth by Robert Foster, and it seems to me logical that the creature came from the sea.
http://encyclopedia.thefreedictionary.com/Sirannon , for general Watcher related observations.
It seems reasonable, since it's obviously a water creature, that at some time the Watcher came up the Gwathlo and the Glanduin, looked around and thought it had found a nice home that, suitably renovated (damming the stream) could provide comfort with a movable buffet feast nearby.
Maybe there were even nastier creatures in the seas that it was avoiding (hee, hee), isn't there some such allusion to dangers of the seas during the Council of Elrond about throwing the ring in there?
If the dam was breached perhaps it would take the hint and return to the sea?