If you are asking about simply giving your cat water, of course fresh water should always be available to your cat. Denying a cat water, although you might think that it is unnecessary as perhaps they urinate almost immediately, animals cannot communicate to us what they need, all they know is what they want so please do not stop providing water, it is essential to keep animals properly hydrated.
If you are referring to fluids as suggested by a vet, here is some information about what these do and why they would be used.
Fluids can be administered to a pet in 2 ways:
- Subcutaneously (under the skin).
Intravenous fluids are usually used in kidney disease when a cat has acute (sudden) kidney failure or when a cat with chronic renal failure starts decompensating. This is a rescue method.
Subcutaneous fluids are used at home daily, every other day, or even twice weekly to slow the progression of chronic renal failure and improve quality of life. As a rule of thumb feline subcutaneous hydration will prolong life.
With cats in kidney failure it is about balance. It is important to provide treatment to prolong a good quality of life. Many vets would use subcutaneous fluids in pets that tolerate them well and are doing well otherwise.
If your cat is vomiting, not eating, and losing weight while on subcutaneous fluids then yes I think you might be prolonging the inevitable. If your cat has a good quality of life and tolerates fluids then it is appropriate to continue this treatment. Your veterinarian knows the details of your cat's case and medical history and a vet will always be honest and advise you if you pet is in pain and should be put to sleep, they will only recommend this if it seems cruel to keep your cat alive.