What a great idea! Since I live on the desert where the dogs get really HOT in the summer, I got the largest, deepest, most heavy duty hard plastic baby pool I could find.
It is filled May first and drained October first. To keep it clean I use a mini chlorine float designed for spas.
When I drag that thing out in the spring, my dogs go wild with anticipation. They have spent countless hours rollicking around in that pool - which is now over ten years old and still holding it's own. My neighbors even send their dog over to join in the fun once in a while. After a "play date" day, the dogs sleep like the dead!
One of the best "doggie" purchases I ever made!