to add to this, each canteen is .5 lbs of weight with or without water. I was thinking though before Reventage stated, that empty it have .1, with 1 charge still .1, and then with each additional charge it gets .1 ending at charge 5 with .5 weight.
Perhaps the weight doesn't have to change.. but half a pound in a canteen as Reventage says, should definately quench thirst, and it takes about one and a half canteens to do so if you've taken the stat damage from dying of thirst.