Not to sound like a broken record by preaching their products again, but check out the stuff by No-Rosion. They make a coolant cleaner that will etch & coat the cast iron and prevent the coolant from rusting the block from the inside.
I've been using their products for years and use water in my cooling system. I know it works for I need to drain my system out each winter due to having no heat in the garage. Water comes out looking exactly like it went it.
Highly, highly recommend their products.
https://www.norosion.com/tech_coolant.htm