My last cat, Bob, was just recently put down and he had the exact same symptoms. Massive water intake (followed by massive urinating everywhere except his litter box), an unusual gait and total lethargy (even more than normal cats). We took him to the vet and they diagnosed feline diabetes. Due to his advanced age, and the twice daily requirement for insulin, we euthanized him. This is the first time in over 20 years that there hasn't been a cat in my life.