The invention provides a floating water surface cover module, comprising a rim (1), and a shallow dome-shaped cover (2) extending from the top of the rim (1) and formed with a vent (3), the rim and cover being formed with shaped air-filled air-tight cavities (5) spaced around the rim (1) to provide buoyancy, the rim (1) and the cover (2) being configured so that one module will nest within anotherto form a stable stack.