I want to make crêpes, but I have run out of milk. I do have all the other ingredients I need. Can I somehow substitute the milk by water or anything else?
You can substitute but don't expect them to be exactly the same, of course.
The fat, protein, and sugar in milk all interfere with the flour and egg protein binding in the crepes. Milk will produce a more tender product.
Also expect the flavor to be a little less luxurious without the fat and sugar. You can use vanilla or replace some of the oil with butter (if your recipe doesn't already call for them).
Powdered milk or evaporated milk are good to have on hand for when you get in situations like this.
You may substitute water, yes, but be sure to also add in a flavorless oil, egg and some Vanilla. The water should only be a bout a cup or so so as to not dilute the batter to much.