Diabetes is one of the chronic illnesses we see most often in our cat clients. Excessive water drinking with increased urination is often one of the first signs a pet owner may notice in a diabetic cat. Diabetes can sometimes be managed by a low-carbohydrate, high-protein diet alone, the "Catkins diet." But, it often also requires insulin. Below is a great article from Dr. Aimee Castor on diabetes in cats. 

https://sites.google.com/site/felinehealthcare/home/diabetes-in-cats