Ditto on the anit-freeze. If you are running straight water, with the system drained, you can have rust forming because the inside that is now exposed to oxygen. Rust needs two things to form, moisture and oxygen. (sure rust will form with just water, but maybe not as fast)
I try and run my cars every so often up to temp. Keeps me happy hearing them and keeps them limber.
Wonder how often I have read that a car sitting deteriorates faster then a driven car! Well it's a good excuse anyway. LOL