Water is the most basic of necessities. Living on half-rations of water has no stat effects. A character can survive indefinitely on quarter-rations of water, but suffers a -2 penalty to saves against poison, heat, fatigue, and some other hazards. A character that drinks a full half gallon of water each day receives a +2 bonus of the same saves.
A character living on eighth-rations of water makes saves as often as they would if they consumed no water, but the save DC does not increase after each check.
Dirty Water gives no benefit for drinking a full half gallon each day, and may be irradiated.