I visited Wadabori outdoor pool during the last week of the season. It’s super old but beautiful, with two sides facing a large, forested temple. It’s also close to Zenpukuji river. Highly recommend it, except that visible tattoos and piercings are not allowed. Not cool!
I’m also confused by the other typical Tokyo pool rule that prohibits sun tan lotion. Ever heard of skin cancer? Apparently not, since the lifeguards are often the shade of burnt toast.