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) In 2059, the Tenth Doctor surmised that this made humans 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)

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. (TV: The Fires of Pompeii)

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

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