There is not a definite answer on this one.  Some will argue and say that wet cat food is better than dry, while others think that the opposite is true.  Some feel that they are equally great and feed their cats both.  Really, it is a personal preference of yours and your cats.  If you are feeding your cat only dry food, Purina does recommend that one cup of water is drunk for every 10 pounds of weight in a 24 hour period.  So, therefore, a 15-pound cat would need 1.5 cups of water every 24 hours.