Keep in mind that cats are not SUPPOSED to drink a lot of water. Descended from desert cats, they have a low thirst drive and are set up to get the great majority of their moisture from their FOOD. This is one reason why feeding dry food is such a very bad idea. Feed raw or canned food (while providing clean water at all times) and you won't have to worry about your cats getting enough water--they will be getting it as nature intended, from their food. And no need to buy expensive water fountains!
More on this here:
Feeding Your Cat: Know the Basics of Feline Nutrition :: healthy cat diet, making cat food, litter box, cat food, cat nutrition, cat urinary tract health
Edited to add that I would worry if I saw my cats drinking much water.