My local zoo has a few venting machines scattered in the far reaches of the zoo so you don't have to backtrack to the little restaurant they have.My problem with vending machines is that it is a very commercialized concept. In a real zoo from my experience the water are free. Forcing guests to pay for water is a very greedy and theme park like idea, not something you'd expect in a zoo.
I think one or two drinking fountains too if I remember right.