Iceland's Blue Lagoon is a shallow lake, geothermally heated, using the run-off from the nearby power station. It varies between a couple of feet and body height in depth, and has a complex, volcanic rock shore full of turns and bends. The water is an eerie, cloudy blue, and as hot as bath water.
I'd love to show photos, only we couldn't take any because we visited at night, and spent a contented hour in the dark, hanging out in the lagoon, sometimes wading through steam that meant we couldn't see more than an arm's length away, sometimes - in the more open areas - able to see a full half of the lagoon, with areas in shade, areas like up, columns of steam over the hotter water. If you wanted to cool down, the air was only a few degrees so you could just stand up, but it was best to keep down. Gorgeous.
Something I got a kick out of was how they prevented people from mucking around. It's supposed to be a tranquil place--I can't imagine they want people standing on each other's shoulders, or, uh, making use of the dark, secluded corners.
At the front of the lagoon is a powerful spotlight, perhaps a metre across, mounted right in the middle, high up, on top of the main building. Usually it was kept pointing ahead and up, away from the water. But if people were seen playing around, the spotlight would be directed right at them. Lighting someone up in the dark means one thing: Everyone else can see them. It was a wonderful way of providing people with freedom and privacy, but selectively activating social visibility as behaviour moderation, when required. It was a gentle panopticon, using the tut-tut of disapproval in place of the inspector's gun.