Altair water is a beverage exported from the Altair system.
While under the influence of Captain Spock's katra in 2285, Dr. Leonard McCoy ordered an Altair water from a bar he frequented on Earth. (TOS movie: Star Trek III: The Search for Spock)
In 2293, Spock ordered an Altair water when he and Alexandra Tremontaine visited the Notes on the Water café on Alonis. (TOS novel: The Fire and the Rose)