I wouldnt put anything in their water unless they are ill and not drinking. Then it would be PURE not from concentrate fruit juice. I have only ever done this with a small amount of carrot or apple juice when Barney refused to drink on his own for days after his neuter, the only way I could get liquids into him was to syringe him dilute carrot juice and when he got a bit better I put very dilute juice in his bowl. Luckily he has a really strong stomach so the extra sugar didnt cause a problem, but its not reccommended. I only did it as he could have died otherwise. It took a good couple of weeks to wean him off it after, he wouldnt drink normal water after having it flavoured like that.
I would not use cranberry juice because commercial cranberry juice is always diluted and has either sugar, sweetener, or both added, citric acid usually too.