Water pool was a variation on pool played in a bar in Paradise City on Nimbus III. The game involved a literal pool of water for at table in which the balls floated and made glass-like sounds when they made contact with each other. It was perhaps somewhat ironic that the game was practiced on the desert planet of Nimbus III where water was quite scarce.
James T. Kirk made alternative use of the pool table to defeat an aggressive three-breasted female feloid alien who had attacked him during his attempt to liberate Sybok's hostages on Nimbus III. He literally throws her into the water of the pool table, causing her to either pass out, or possibly die. (Star Trek V: The Final Frontier)