I don't think supplementing the water will really help. Plain fresh water if it will drink is plenty. Give it a few days and see if it will survive, I know that's not really what you want to hear but cats can really do a number on little creatures. A quiet, excape proof, warm cage with a place to hide to reduce stress (avoid handling unless treating the injuries) may help it to recover. If it seems to be able to move around on it's own in a week or so, then start thinking about feeding it. Most snakes can go a while without food so don't worry about that for now. If it's really small, feeder guppies might work. I've used those for newborn water snakes.