Water

 was a liquid that made up 60% of a human (TV: Tooth and Claw, The Waters of Mars) and 70% of Earth's surface. (TV: The Fires of Pompeii) It contained hydrogen atoms. (TV: Arc of Infinity)

In 2059, the Tenth Doctor surmised that humans being predominantly water made them the "perfect host" for the Flood when it infected Bowie Base One's water supply. The Flood also wanted to get to Earth because of its abundance of water. (TV: The Waters of Mars)

Water was a prized liquid for those who lived under the surface of Ravolox. (TV: The Mysterious Planet)

Water was a seen as a source of great wealth for the Martians. On Mars, great warriors were often entombed in blocks of ice. (PROSE: The Dying Days, AUDIO: Red Dawn)

Rain on a human colony planet appeared to be living water. (PROSE: Pitter-Patter) Kar-Charrat also had sentient water on it. (AUDIO: The Genocide Machine)

Kromon was described by the Eighth Doctor as a "world of water". (AUDIO: The Creed of the Kromon)

Water could form on the walls and ceilings of structures such as the Doctor's TARDIS as moisture during the process of condensation when the outside cooled but the inside remained warm. (TV: "Five Hundred Eyes")

Water was what caused floods. (TV: Before the Flood)

Effect on species
A Thermortian Sand Worm's organs would collapse to sludge if it consumed water. (COMIC: The Forgotten)

Skullions didn't drink water as it "burned" them. They preferred citrus drinks such as orange and lemon juice. (TV: The Man Who Never Was)

Seed pods were resistant to acid, but vulnerable to water. (TV: The Seeds of Death)

A small amount of water was enough to kill a Pyrovile foot soldier, as they would solidify and then shatter. (TV: The Fires of Pompeii)

Water was fatal to the Vrakons. (COMIC: The Devil's Mouth)

The Kromon required a constant supply of water to live. (AUDIO: The Creed of the Kromon)

As a drink
Captain Erisa Magambo gave Donna Noble a glass of water to help combat the dehydrative effects of the Lodestone. (TV: Turn Left)

Other uses
Water could fuel a Gallifreyan fusion booster. (TV: Arc of Infinity)

Behind the scenes

 * Tom Baker states he had an argument with a producer about how he chose to hold a glass of water during the filming of an unnamed serial. Baker had taken to holding the glass up to his ear briefly, and when questioned about this stated that there is an old Gallifreyan tradition "as part of hospitality" where you listen to a glass of water before drinking it. However, as this is not stated in any Doctor Who story or serial it is to be considered a character choice and not valid. (HOMEVID: A New Body at Last)