(Please note many aspects of this FAQ apply to Dogs as well as Cats, but for its purpose the faq refers to cats specifically).
People don't realize just how important "Wet Food" is to a cat's diet. Cat's get a large portion of their hydration from their food. A dry kibble just isn't sufficient to the task. Dry food was actually created as a form of convenience for humans when feeding their pets. It's the least optimal form of food you can feed your cat.
For example; a cat needs to hit about a 75% hydration rate (minimum) to stay properly hydrated. With dry food, they'll only manage about a 60% hydration level. Add to that a moving source of water such as a fountain and you'll get a boost of about 5% putting your cat at a 65% hydration level.
You're possibly saying "But my cat drinks so much water!!" at this moment. But statistically, it isn't necessarily true.
Most cats DON'T drink enough water. The amount of water they actually take in with each visit to the water bowl is very small.
What happens in these cases is most cats will stay low level dehydrated for months, even years on end. Not enough to be noticeable on the exterior, but enough to cause health issues to crop up.
Crystals, IBS, Impaction, mega colon; these are all common issues that low level dehydration will eventually lead to in most instances.
So, it's important to find some type of "wet food" that your cat can enjoy eating.
The ideal amount of "wet food" is approximately 3 ounces per 7 pounds of body weight daily.
As to what type of food? It's important to find a healthy food, preferably a human-grade food. most pet foods we grew up with are made from meats and grains that were rejected for human consumption. This plus the high heat cooking process means they have poor nutritional value. Add to that the grain and corns for filler, and you've got a terrible diet for your cat.
Remember, cats are obligate carnivores. This means they're carnivores (meat eaters). They lack the digestive track length that herbivores and omnivores have.
Normally, a carnivore will kill it's prey (say, a seed/grain eating mouse, in this case). During the process of feeding they'll eat the intestines and the stomach contents. In this manner, they eat the partially digested grains and vegetables and thus get the vitamins they need from those food groups.
So it's important to go for a low or no-grain diet with cats, something high in protein and low in ash content. Cats have a very difficult time digesting grains. In many instances you don't want to use any grain at all. True, a small amount of grain can help as a binding agent, but it's best to have none, especially if your cat has digestive issues.
If your cat won't eat wet food, then try human chicken, meat or fish. But remember that human packaged poultry, meat and fish don't have all the vitamin requirements that a cat needs. You'll have to go with a vitamin supplement to ensure your cat is getting all the neccessary vitamins and minerals that a cat needs.
Also, remember that a cat will eat not only until it is full, but also until specific vitamin and mineral needs are met. When those needs are met, the brain sends a signal to the "hunger center" that lets the cat know it is full in EVERY way, and should stop eating.
This is why you see so many cats on poor-nutrition diets that are overweight; they keep eating despite a full stomach because they're vitamin/mineral deprived. If this diet issue isn't resolved quickly, it can lead to a behavioral-obsessive-eating disorder.
This is yet another reason why a good quality, properly balanced diet food is important to your cat.