Always mix some water into their food, even into the canned food. The more water they ingest the better their kidneys keep working. been there...several times.
We also give sub-cutaneous water injections to our one kitty about 2x week to keep her from any downward slide, since she has damaged kidney function already from a crisis several years ago. This keeps her doing well. But I always mix a tablespoon of water into each of the kitties meals anyway.

Personally, I have never been impressed with the ingredients in the 'special' kidney health diets that vets sell for high prices.