Dirty water

 is the most common water in the game, and is drawn from the tainted lakes and rivers of Fallout 3. Unlike purified water, this will give you radiation, which can be cured for a fee by a doctor, by using Rad Away, or by using your infirmary.

Fallout: New Vegas
As an infirmary is not available in Fallout: New Vegas, consuming contaminated water is much more irksome than in Fallout 3.

Moreover, with hardcore mode enabled it can be difficult for players early in the game without a canteen to find adequate quantities of purified water. Drinking dirty water is preferable to taking the penalties from thirst