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. It is still worth drinking, however, as the cost of hiring a doctor to purge radiation is significantly cheaper than the higher cost of buying purified water in large quantities.

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.

Mass purified water can be used to purify dirty water. However instead of using bottled water in large quantities you may find it preferable to fast travel to the Goodsprings Source, the NCR farms outside of New Vegas or Kings' pump in Freeside (free after Bill Ronte is sober) to drink out of a pump.