This happened last summer. I was camping for a week at a music festival with a group of friends, 4 guys and 2 girls. Masturbation during this time was next to impossible, as the only facilities available were porta-potties, which got incredibly hot in the 90+ degree weather. By the fourth day I thought I was going to explode.
One particularly hot afternoon the whole bunch of us decided to go in the lake together to cool off. We grabbed a bunch of air mattresses and headed out into the water, past the ropes. We were all just floating there together, far away from everyone else, when I suddenly popped a huge boner. It was clearly visible through my swim trunks, so I rolled off the air mattress into the water, with my chest and arms still stretched across the mattress. I thought the cool water would make my erection go away, but no such luck. If anything it was getting harder. I needed some relief.
Now something you need to know about this lake is that the water is very murky. You can only see about 8 inches into it; everything below that level is completely invisible. I found myself thinking about the previous summer at the beach, when I had attempted to jerk off in a different lake, but had failed because the water was just too cold to sustain an erection. Of course, that time I hadn't been anywhere near as desperate as I was this time, and the water here was not nearly as cold as the other lake had been...
I slid further into the water, until it was up to my chin, holding on to the edge of the air mattress with one hand to keep it from floating away. With the other hand I carefully reached under the water and untied my trunks, pushing them down almost to my knees, then looked around at my friends. Nobody had noticed a thing. They were all still talking and laughing. By now my cock was throbbing so hard I could hardly stand it, so I wrapped my hand around it and began to pump.
I went slowly at first, until I was certain that the movement of my arm was not detectable above the surface, and then I really started going at it. Given the circumstances, with all of my friends within about a 15-foot circle of me, I needed to finish rather quickly...but considering how horny I was, I didn't think that would be a problem!
I probably stroked for about 5 minutes, somehow maintaining conversations with the people around me, when I started to feel like I was getting close. I doubled my efforts, knowing that I could only keep from getting caught for so long. Just as I was reaching the point of no-return, my buddy 'Mike' let go of the double-size mattress he was sharing with another guy and swam over to grab my mattress. My first impulse was to stop, but it was too late. My dick started to spasm, and I pumped about 4 shots of cum into the water. As soon as I was done, I pulled my shorts back up and nonchalantly began paddling my air mattress away from the evidence. Thankfully everyone followed me before my sperm floated to the surface. I couldn't believe it. Somehow I had managed to jerk off in the middle of a mixed group of friends, and nobody had noticed a thing...in fact, nobody would ever have known if I hadn't told my best friend a couple of weeks later. ('Mike,' however, will never know that he was less than a foot away from me when I jacked myself to orgasm.)