I have never had a problem with giving cats regular 2% milk without mixing it with water. But it absolutely does need to be fresh. The only problem that I have with feeding cats milk is that it is quite fattening. So in that aspect it might be a good idea to add some water to it. I don't feed milk to cats except the pregnant or nursing moms or really think cats who could use some extra fat. If the cat will not drink water, maybe you could start with adding small amount of water to the milk and then gradually increase the amount of water until the cat will tolerate water alone.
However, if you don't usually feed this cat milk you should introduce it slowly. Small amounts at first and then a little more each day. Any drastic changes in a cat's diet might be upsetting to their stomachs.
However, if you don't usually feed this cat milk you should introduce it slowly. Small amounts at first and then a little more each day. Any drastic changes in a cat's diet might be upsetting to their stomachs.