Freze Plug

Antifreeze lowers the freezing point,raises the boiling point and reduces the calories per gallon that the mix absorbs per degree of temperature change. However the higher boiling point more than makes up for the difference. If corrosion were not an issue ( yes antifreeze has inhibitors and pump lubricants as well) and the temp would not go above boiling ( at system pressure) straight water will carry more heat. Since corrosion IS a factor and freezing IS a factor I would not recommend ever leaving a cooling system with just water.
The core plugs, if they do pop out when a block freezes, usually do so because of distortion of the openings.