Abstract: The surface activity (more especially waves) of a body of water is reduced by providing a succession of booms at least partially surrounding said body of water (preferably moored so as to be substantially stationary) so that the surface of said body of water (preferably a major proportion or all of said surface) is substantially isolated by the presence of said booms in relation to the surface of water peripherally external with respect to said booms and the surface activity of said body of water is reduced by the distribution on the surface of said body of water as thus isolated by said succession of booms of a material at a cryothermal temperature at or below the freezing point of the water that preferably is in the form of a cascade of ice particles. Preferably surfaces of succession booms that face the so isolated body of water present material at a cryothermal temperature that is in contact with the water surface.