Near the bow of a cruise ship at sea, under a full moon. On this particular ship, there was a large observation deck area right up front, and at night, this part of the ship was never lit. It was about 2AM and there was no one around. Wifey and I couldn't sleep, having over-indulged at the midnight buffet. We were taking a walk around the upper deck when the mood took us. Actually felt bold given the hour and lack of lights, so got totally undressed and were going at it. Shortly after we were spent, but still enjoying each other, another couple wandered onto the platform - with the same thought perhaps?! - just grinned at us and kept walking. We got dressed and went to our cabin. I wanted to hang out hidden near the bow, convinced the other couple was going to pick up where we left off!
Once we got back to the cabin, we were re-energized and enjoyed round two out on the balcony.