A little harder to estimate the liquid needs of cats, but mine choose primarily to drink rainwater, with tapwater being second choice.
Heck, aquarium water, dead Sea Salt and Epsom Salt water are even preferrable. I suspect it may be one of their primary mineral sources.
The gut contents of mice are their sources of probiotics, and fresh mice are higher in water content than catfood by far. Dried catfood is meant to be soaked in water before it's feed to the mews. Have you tried that?