What we do with our cat to stop him being dehydrated is syringe water into his mouth a few times a day, it's no hassle, when i check his BG i give him water at the same time.
We try to make sure he gets about 330mls of fluid into him daily, this includes the moisture content from his food.
We also add 25mls of water to his breakfast and supper.
If you take away the moisture content from his/her food eaten daily then depending on your cats weight syringe the remaining amount of water slowly into their mouth a few times a day.
They say a cat should have 50/60mls of fluid per kilo body weight daily.
We have been doing this with our cat for a good few months now because after a vets visit she said he was dehydrated and doing this method has worked well keeping him hydrated, plus his coat is a lot more healthier.