The Watcher in the Water lived in a lake that that was described by J.R.R. Tolkiens as "... a dark, still lake" which was created by the damming of the Sirannon river, which was located on the west side of the mine of Moria.
The Watcher in the Water was one of the many dangerous obstacles standing in the way of the Fellowship.
According to the most ancient tales, Melkor, the most powerful (and the most feared) of all the Dark Powers and the Valar, in his fortress of Angband bred many terrible creatures for which there were no names in the Time of Darkness before the Valar kindled the Stars. In the following Ages, these creatures were a bane on land and in the dark waters to those who lived peacefully in the World.
Some of these beings of Melkor survived in the deep places of the earth in an ancient, dreamless, undisturbed sleep even until the Third Age of the Sun. It is told that when the fiery Balrog was loosed upon Moria, another being came out of the dark waters beneath the mountains. This was one of the great Krakens, a massive creature with many tentacles and a slimy sheen. It was luminous green and had an inky stench emanating from its foul bulk. For many years, it lay patiently in the black water beneath the mountains.
Eventually it came to the clear water of the River Sirannon, which flowed before the West-gate of Moria. There it built a great wall in the riverbed and made itself a black pool, hideous and still. This being was the guardian of the West Gate that none could pass without challenge. It is not known why the Watcher guards the West Gate: It can be speculated that it is put there by the Balrog ruling Moria. Or it could've just been an animal that was defending its territory and had no connection to Morgoth of the Balrog.
During the time when Moria was being invaded by Sauron's Goblins it built a dam in the river that was near the West Gate of Moria. During the attempted recolonization of Moria by the Dwarves, the Watcher killed Oin when his reconnaissance party visited the West Gate. The Watcher later attacked Frodo, but the Fellowship was able to injure the beast and escape. This enraged the Watcher and caused it to tear down and barricade the Westgate of Moria, thus trapping the Fellowship inside. It might also have wanted to trap the Fellowship inside Moria and force a confrontation with the Balrog.
It is likely that the Watcher detected, or was drawn by some evil power flowing from, the Ring and grabbed Frodo due to that call. However, in the film Peregrin Took dropped a stone into the water, which may have caused it to attack, for it knew there were intruders. In addition, in the novel, it attacks when Gandalf opens the Mithril-adorned door of Moria. Perhaps there was some charm on the entrance that would alert the Watcher when it was stepped on.