In humans Diarrhea is a sign of an illness possibly. But since cats are not really affected in the same way as humans it is a sign that either you kitty has picked up an internal parasite or there is something more wrong with its health. I found that cats that are pretty young under a year or so tend to dehydrate really fast. Adults do the same but the younger kittens go really fast. I would say if you kitty has had this for a while I would get it in to the vet for other medical attension. To save a trip bring a stool sample with you also that will be the first thing that the doctor will want to check. Good Luck I hope you kitty starts to feel better soon.