Both or none :)

Water (clean water!) is good to keep hydrated. Drinking only milk for your hydration would mean quite some calories.

(Fun fact: In the ancient world and middle ages water often wasn't safe to drink so most of the people drank beer which was made safe via its fermentation. It only had little alcohol, but a lot people drank beer early in the morning and even children did.)

On the other hand milk has a lot of good things. It's relatively low on fat and high on protein. Protein is often lacking in our food intake in the civilised world. Milk contains a lot of calcium which children need for their growth and with is needed for bone health especially for the elderly. You can ingest other dairy products: butter, cheese, yoghurt,

One problem of milk consumation are allergies, but the foremost problem is lactose-intolerance. In short: thousands of years ago there was gene mutation, probably in Europe, and that's why most Europeans can digest cow milk and people from other parts of the world might not. Even with those "mutants", the ability to digest dairy products decreases if you stop drinking milk after your childhood.