We've been using that same kind of waterer - the 2 gallon double walled galvanized metal kind - since September, and we have no signs of rust. We have pretty hard water here, too. In non-freezing weather, we change the water every night when we are putting "the girls" to bed. In freezing weather (like now!), I've started using a smaller plastic waterer to swap out with the metal one - when one starts to freeze up, I bring out the other one and bring in the freezing one to thaw in the bathtub.
When you say they rust "right away", how soon do you mean? I would think rusty water wouldn't be very good for any animal to drink. Can you have the pH of your water tested, and treat it(naturally, not with chemicals) to make it less prone to causing rust?
Hope this helps . . .
NancyB