Purified water

Purified Water is (as the name suggest) water that is completely free of radiation and so it can be safely consumed without harming the player. In addition, it also heals far more then Dirty Water does by awarding 20 Health instead of 5. It's relatively rare in the Wastes, but can be requested by Wadsworth, the robot that takes care of the players house in Megaton or Tenpenny Towers (depending on player's choice of quests).

Purified Water can be given to Micky outside Megaton or Carlos outside of Rivet City for positive Karma; this introduces the possibility of infinite Good Karma, as neither will ever stop asking for more water.

There also seem to be random occurances 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 sustinance).