This is Thirroul Pool, on the Illawarra Coast, south of Sydney, on a very stormy day. It's an olympic sized pool at Thirroul Beach, which is tidally filled with seawater. You can't actually see the beach because of the wall at the left of the photo. The first thing that alerts you to the fact that it's not a conventional chlorine pool is the sign announcing the times when the pool will be closed due to low tide.
I had a great swim in the pool, then walked outside and had a swim, between the flags, from the beach. I even managed to catch a couple of (very gentle) waves.