The Swimline 8-Foot Double Water Tube is made specifically for use with your rectangular or oval pool cover to eliminate stress on your pool and the cover. It's 33% heavier than other water bags, making it super durable - plus its valves are easy to fill up with a garden hose. This Swimline water tube is best used with in-ground pool covers. Covering your pool with a winter cover keeps out dirt and debris, and this water tube helps keep the cover in place. We are an Authorized Swimline Swimming Pool parts dealer! Swimline is a family owned and operated business founded in 1971 and is the leading manufacturer of high quality pool products. At its inception, Swimline was the largest manufacturer of in-ground and above-ground pool liners, covers, and all related accessories.