Cats are known to be not so great drinkers! Cats, in general, get most of the liquid they need from food. and if they will need to drink, then water is what to go for since it is the most adequate beverage for cats, no surprise here!
Milk is also to be classified as food, not a beverage! It contains many nutritients, such as albumen or calcium, and it is specially good for kittens. However, cow milk has a different composition than those of the mother cat's milk., it is a known fact that many cats cannot tolerate it and therefore are exposed to diahrrea (some will even vaumit it). If this happens do not give them any type of milk anymore, and even if you water it, it is useless. As a rule to be known, milk is worthless as a beverage.
Conclusion: Water, and only water for cats.
The homemade cat food website.