Worlds without water should have a minimum colonization cost as life is pretty much impossible without it. Besides needing to drink it, you need it to grow food, for cooling purposes, for washing and sanitation, and in a large number of industrial processes. If it is not present in quanity you have to go mining it from asteroids or comets and the amount you would need is non-trivial. 1 million people need over far more than a million litres of water per day, and no recycling process is perfect. Admittedly 1 m litres is only a 10 m x 10 m x 10 m cube but still that would amount to a fair amount of asteroid mining, and something that would grow quickly with time.