Does anyone have suggestions for bodies of water that allow dogs but aren't too cold?

This is probably a pipe dream, but I would love to find a good spot for my little minpin to swim. But there are a couple catches- it can't be super cold water because that's tough on his arthritis, and ideally it's not super busy with other dogs because he's fear aggressive toward dogs. I wouldn't let him off leash and I would be with him in the water the whole time. And we have some weekday flexibility to go to places when they're not as busy. If anyone has any ideas, please let me know! Swimming is such amazing exercise for my orthopedic nightmare of a dog, but we haven't found a good spot yet.