I second the drain plug heaters. Heat rises....They work better than the floaters. The water seems to be more cooler than frigid with the drain plug heater. Just make sure your seal is good, they have a tendency to wear out before the heater does. I wrapped our tank with insulation and duct taped all of the outside which helps. Covering the top half of the tank would help too, just my two cents.
Horse haven't chewed the duct tape yet...knock on wood!