Abstract: Apparatus for preventing or retarding the freezing of water on a surface of a body of water, such as in a stock tank, includes a tank containing a quantity of volatile fluid, which functions as a heat pipe to distribute heat from a lower portion in the body of water to the surface to prevent ice on the surface from forming thereabout. The tank is ballasted to float just at the surface of the water, and additionally is anchored to control the positional location of the tank in the body of water. In one embodiment, a heat containing collar is employed surrounding the top portion of the tank, and within which the tank is free to move to contain the distributed heat. In another embodiment, the entire surface of the water in a stock tank is covered with an insulation layer having holes of spaced relationship to receive a plurality of heat pipe tanks.