Gollum's water, also known as the dark lake, was a body of water that existed within Gollum's cave by which Gollum lived alone for many decades.
Deep within the Misty Mountains, this lake was formed by water that had seeped through the rock and formed the cave. The water was wide, deep, and piercingly cold, yet fish somehow lived in it. An underground stream flowed from it to the outside.
History
Gollum lived off of the lake and its fish. He paddled a homemade boat back and forth between the lake and a small island at the center of the lake, which was also Gollum's home. The Great Goblin would occasionally send goblins down to get fish, and they were in turn eaten by Gollum. The goblins, however, did not seem to suspect Gollum's existence.
The lake first appears in The Hobbit in the chapter "Riddles in the Dark", as the meeting place of Gollum and Bilbo, after the latter discovers the former's lost "property:" the One Ring. There, the two engage in a riddle-game, which ends when Bilbo asks Gollum what is in his own pocket. Gollum then discovers his ring is missing and finds that Bilbo has it.