Well, not exactly a swimming pool but rather a wave pool. I found this at kilian-nakamura.
One reader comment to the photo above was rather amusing: "...Americans would not be able to handle that kind of closeness. There would be fights. It is a testament to the manners in Japanese society...."
Then the retort: "I agree with Clayton. Americans would not be able handle that kind of stupidity..."
See the below You Tube of the spectacle once a wave rolls by: