This past month, I visited a nice lake in Vermont with an established nude swimming area. There is a small cliff that runs right into the water, with a variety of little coves and ledges, above and below the surface of the water. I'd been there many times before, so I immediately got naked and in the water.
Swimming around the ledges, I found a little indent in the cliff, with a convenient shelf to stand on underwater. From here, I'd be able to see anyone coming close before they'd see what I was doing. There was even a little overhang in the cliff above, so I couldn't be seen from the top. The beauty of the day, the cool clean water, and the good feeling of nudity got to me, and I began stroking.
A few rafters and canoers in the distance, and a crowd but a few feet away, never saw me. Usually, when I get to climax quickly, it is less intense than when I take a long time, and this orgasm started like that. But when I ejaculated, I realized that I'd gotten away with a public orgasm, and a 'second wave' of pleasure hit me intensely, even though I was unloaded!
Seconds later, a couple paddled around the cove on a raft, not six feet away, my dwindling penis still flowing.