The order of Sand - centered around the city of Kantaras, was created to worship the great sands, and despises all water.
They still need to consume water, as they are alive, however they do so in secret and away from prying eyes.
Reminding a Sand worshiper that they require water as a means to survive is a great insult, and many will chose death over exposing this hypocrisy of their own faith to others.