Water is a chemical compound that is the most common liquid in the Milky Way Galaxy, and is crucial to survival for most carbon-based lifeforms, as well as some silicon-based beings. Water is a compound of two elements, hydrogen and oxygen. Water also comes in a frozen form, known as ice.
In the year 2268 there were nine varieties of water sold at a cantina located in the Gorn city of Tzoryp. Two of the varieties included carbon water and frozen water. Cervantes Quinn wanted the carbon water poured over broken chunks of the frozen water. Two glasses of this variety of water cost him six szekets. (VAN - Declassified novella: The Stars Look Down)
In 2367, Governor Gosheven called the water in his grandfather's aqueduct system the shared years of blood and sweat from all who sacrificed settling the Tau Cygna V colony. (TNG episode: "The Ensigns of Command")
In 2369 the water supply of the Dracon monastery on the planet Riat was infected by an algae which had been irradiated by solar flare activity. When the monks at the monastery consumed the water they underwent a mutation which made them violent and cannibalistic. Doctor Beverly Crusher of the USS Enterprise-D was able to cure the monks of the infection. (TNG comic: "Light of the Day")
In some regions of the Delta Quadrant, water is a scarce and valued commodity, traded amongst the Kazon and the Talaxians. In 2371, Captain Kathryn Janeway used water to bribe First Maje Jal Jabin into revealing all information he had about the Ocampa and the Caretaker. (VOY episode: "Caretaker")