Every year we have a zillion posts on this subject. Nothing really stops it freezing. I empty mine at night, and then refil with fresh water every morning. Much easier than trying to defrost, and kinder to the plastic waterers, which can get brittle and crack.
PLEASE dont be tempted to put the water inside the actual coops at night. It increases the humidity and condensation in there, and can be quite nasty for the chickens....damp as well as cold.