Actually, using Copper Sulfate is very common. Farmers add it all the time to keep water clean, Copper Sulfate is excellent at keeping water free of bacteria and algae etc.
I assume OP has a large watering system with parts that can't be totally cleaned. Large tank and thin piping maybe? If you follow the correct dosage I have never heard of it being a problem. I always use Copper Fittings in my water builds for all my animals, cat, dog, chickens, goats just for this purpose, to stretch out the times between cleanings or to stop super gross things from growing.
Edit: ooooo just researched it. I had no idea Copper Sulfate was also used to stop the spread of canker, through water atleast. Interesting but makes sense. Still, if you are doing the correct amount I don't see why it would be bad. Lets be honest, if the water isn't to the cats liking he will stop drinking pretty quickly. Most my cats won't drink water that has even sat out for longer than a day :/, cats.