Purified water

Purified water is water that is completely free of radiation and as such can be safely consumed without harming the player. In addition, it also heals far more than dirty water does.

Fallout 3
It's relatively rare in the Wastes, but can be requested from Wadsworth or Godfrey inside My Megaton house or Tenpenny Tower suite respectively, at a maximum of 5 per week.

Purified water can be given to Micky outside Megaton, Carlos outside of Rivet City, or Willy in front of Tenpenny Tower for positive karma. This introduces the possibility of infinite good karma, as they will never stop asking for more water.

There also seem to be random occurrences in the wastes of water beggars. If you deny certain ones water they request to be shot in the head (for negative karma, of course...just not for denying them sustenance). In another random occurrence, you may encounter two groups fighting over a fridge stocked with 10 purified water bottles. You can leave them alone, kill them and take the water, or with a successful speech check, convince them to split the water with you.

There is also another random encounter which involves two wastelanders requesting you kill a radscorpion guarding a refrigerator with about 7 bottles of purified water. They will then allow you to take all the water.

Fallout: New Vegas
The Mojave Wasteland has multiple natural sources of pure, non radioactive water, and as a result the player encounters clean water more often than in Fallout 3, where most of the water is radioactive. Drinking from a clean water source, such as the pumps at Goodsprings source or Lake Mead, will remove 200 points from the dehydration meter. Purified water can also be bought from several merchants, including the Kings member who guards the water pump in Freeside. The player can also fill empty bottles in The Sink in the Old World Blues DLC.

The effects of purified water are modified by the Survival skill, the higher your skill, the more HP and H2O it will restore.

When playing hardcore mode, purified water is a more refreshing drink than alcohol or soda.

Cactus Water
Cactus water is a crafting recipe. It requires a Survival skill of 30 and the following ingredients: Once crafted the player receives a purified water, not a cactus water, in their inventory.
 * Prickly pear fruit (2)
 * Empty soda bottle (1)

Behind the scenes

 * Each bottle of purified water appears to feature the text "'H²O" hand written on the label on the side of the bottle. This format is incorrect (at least according to IUPAC nomenclature) as in chemical formula numbers are written in subscript (Xy) and not in superscript (Xy). If read literally, the chemical formula H²O would mean there is one hydrogen and one oxygen isotope that contains two neutrons (instead of the standard eight). This would leave the oxygen extremely unstable and radioactive with a half life that would be so small that it would not be measurable.