A method for cooling the temperature of a body of water having a top surface exposed to sunlight comprises distributing balls such that they lie on, and in direct contact with, at least part of the top surface, the at least part of the top surface characterized prior to the distribution by a pre-distribution surface area; The balls have diameters within a range of 100 microns to 3 mm, and albedos within a range of 0.15 to 1.0. The surfaces of the balls are hydrophilic, such that after the distribution a total wetted surface area greater than the pre-distribution surface area of the water is provided, facilitating the cooling. In one embodiment, the body of water is a natural or unnatural lake or bay.