The film takes place in a distant future where water is so scarce and rationed that it is considered an immensely valuable substance, both as a commodity and as a currency in ice cubes. The Templars of Mithra control the water and they destroy worlds that have natural water, leaving the galaxy virtually dry. Pirates dedicate their lives to raiding ships and looting the ice from the cargo holds to make a living.

References:
https://en.wikipedia.org/wiki/The_Ice_Pirates