You can use plain water for making instant mashed potatoes if you don't have any milk and when I run out of milk I use plain water to cook the instant mashed potatoes and it's just as good.
Although the milk gives the instant mashed potatoes a creamier taste.
You can also substitute things such as chicken broth, beef broth or any other broth in the instant mashed potatoes and it will make the instant mashed potatoes taste very good.
You can also add some sour cream along with the broth or some crème fraîche and light cream instead of the milk to make great instant mashed potatoes.
Although if you don't have any of those substitutes you can use plain water in instant mashed potatoes like I do most of the time.