Just out of curiosity, is that with existing antifreeze?
Originally yes because there was anti-freeze in the cooling system when we purchased the boat but we have not added any - since then, we have noted that the temperature in the engine room rarely falls below 10 Celsius and as we live aboard throughout the risky Winter months we can monitor the situation easily. So we no longer use anti-freeze - just Ankorsol and clean water. We have experienced severe frost outside over the last ten years but the temperature inside has always been satisfactory. I guess that is down to excellent insulation. If the temperature in the boat ever fell to less than 10 degrees it wouldn't be just the cooling system that needed protection!
Edited to add that both anti-freeze and Ankorsol are potentially toxic and extreme care should be taken in the disposal of coolant containing either product. Under no circumstances should antifreeze be poured into ditches or drains where it may pollute the environment or poison wildlife.