Dirty water

 is the most common water in the game, and is drawn from the tainted lakes and rivers of Fallout 3 and Fallout: New Vegas. 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