Yes milk is good for hydration.
Milk is actually even better for you at hydration for your body than water because the milk you drink is retained in the body for longer.
When the milk or liquid is retained in your body longer the more you stay hydrated.
Still water is essential for hydration as well but milk is also good for hydration and also helps build strong bones and helps prevent osteoporosis and other joint issues.
Milk contains calcium which is also essential as well as many vitamins and minerals.
So if you want to drink milk when you're needing hydration it's a good drink to hydrate you but you should also drink plenty of water which helps to flush out any toxins in your body and hydrates your body too.