My guess would be raccoons. They are always 'playing' with something in our back yard and pool. If we leave toys or small colorful items outside overnight (kids stencils were a big hit!), they often play with them and wash them in the pool. They also swim laps in the pool when it is hot.

Gary Bogue, columnist for the Valley Times, has many stories about coons; I think I remember hearing one tale about some juvenile raccoons swinging on a clothesline.