Pool Cover Water Bags should be placed around the perimeter of your in-ground winter swimming pool cover. Simply slide these winter water bags through the water bag loops included on your winter pool cover. These water bags include leak-proof valves that are easy to fill with a garden hose. Only fill the water bag half-way, this prevents ice from expanding & splitting your water bag. Pool Mate water bags feature a dual-chamber design, preventing the tube from rolling into your pool during the winter.