I did find this on the interwebs just now:
Does this bottle of unfrozen saltwater make it warmer in the tank water? No! The temperature of the liquid saltwater is just as cold as the freezing freshwater. The idea that the proximity to the saltwater somehow keeps the freshwater from freezing is a myth. However, the floating bottle does agitate the surface which helps prevent and ice skin from forming and horses can push the bottle down with their nose to break a hole in the iced over tank.
Myth busted? Well, sort of… the saltwater bottle does work well to prevent stock tanks from icing over, but not necessarily because of the salt. It works in the same way as any floating object would, by agitating the surface of the water. A partially filled saltwater bottle does float especially nicely in the tank and is more difficult for horses to remove than a lighter bottle.