In the wild, cats get most of their moisture from the animals they hunt and kill, but unless your cat is hunting mice and eating them on a regular basis, chances are it relies on you for all its food and water. An exclusive dry food diet just won't do it either, as your cat doesn't get a sufficient amount of water from the kibble.
So how do you fix this? Well, short of using a syringe to force feed your cat water, you might want to think about switching your cat to a wet food diet, or at least a partial wet food diet.
There are plenty of great canned products available, but there are also plenty of not-so-great products. The best way of telling the difference is to read the food label on the back of the can. The first ingredient should be meat. Don't worry if it doesn't have grains, as they aren’t really necessary in a cat's diet, but above all else, avoid getting canned products which contain fillers like corn and rice. These fillers are only good for bulking out the food (and your cat); it has no health benefit.
If you are still unsure about which cat food brands are of the highest quality, ask around. A recommendation from a trusted friend or a reputable veterinarian can go a long way. Your vet may even have a suggestion of a food product that is similar to your cat's current diet. We wouldn't want kitty to stop eating because of a sudden diet change.