There are a couple of things that I think of when I hear a history of increased water intake and increased appetite. They include the following:
1) Diabetes. Increased water intake, urine output and food intake along with weight loss of the usual signs of diabetes. This is diagnosed by finding elevated glucose in the urine AND the blood. Blood work needs to be done.
2) Cushings disease. Cushings shows all of the above signs except weight loss. In addition, we often see pendolous abdomen or pot-bellied appearance, frequent and recurrent skin infections, poor hair growth, and other signs. Bloodwork usually demonstrates increased serum alkaline phosphatase enzyme levels. Additional blood work needs to be done to confirm Cushings if the screening blood work is suggestive. Your vet can discuss that with you. Cushings is a very complicated disease. Treatment is available, but make sure you talk through all the ins and outs of treatment before you begin it.
3) Liver disease can cause increased water intake, but usually not increased appetite.
4) Parasites can sometimes cause similar signs. A stool sample should be done to rule these parasites out.
These signs really do warrant having a closer look. Start by having your vet do a thorough exam and then do a CBC, chemistries, urinalysis and stool sample. Things hopefully will become clear after that - or at least you will have a more clear direction to head.
Hope these thoughts help. Good luck wtiht his.