Was extreme fun, and I'd be willing to do it again.
So, a bunch of people from my mom's work were getting together at the lake for a day of fun. We had barbecue, swimming in the lake, volleyball, even a good game of tackle football going. There were some younger kids running around, a few teenagers, and the rest were the friends and family of people from my mom's work.
I decided to take a swim in the lake, so I was floating around, when the boss's 13 year old son (We'll call him J.) swims up and asks what I'm doing. 'Nothing, just chillin'' I respond. 'Oh....I got some water balloons over by the pavilion, wanna have a water balloon fight?' Okay, that sounds like fun, so I respond 'Sure' and we run back to the pavilion and grab as many balloons as we can. We go back to the lake, and surprisingly the balloons float, so I pile them around me and start launching them at J. We eventually run out of water balloons, and J, not being a very good swimmer, asks if he can hold on to me for a second to catch his breath.
Not a problem, so he comes up and wraps his legs around my body, his feet rubbing across my dick in the process. I start getting hard, and of course, it starts poking him in the stomach. I feel him starting to get hard too, so he gets off me and turns around in embarrassment. 'It's okay, dude. It happens.' I chuckle, letting him know that I don't care. He comes back and uses me as his lean-post again, and I notice his arms moving ever so slightly. So, being the investigative person I am, I kind of move my hand towards his crotch, and he starts hitting my hand with his as he's pumping up and down. Sure enough, he was masturbating.
'Do you wanna do that somewhere a bit more...private?' I smirked at him as I said it, to tell him I had something planned. He nods, so we swim back to shore, both of our boners subsiding in the meantime, and we head off towards the bathrooms provided by the park. We go into a stall, after checking no one was in there, and both remove our shorts. We start stroking, watching each other, before he asks 'Can I...uh...do yours?' I agree, so I take my hand off my dick, and he wraps his cold hand around it as he slowly starts stroking it. I grab his, since he already took his hand off, and using four of my fingers, I start stroking him.
Not long after, he moans slightly, takes his hand off my dick, and dribbles a little bit of cum out of his dick. I can't take it anymore, so I start power-stroking, and soon I shoot my load all over his stomach. We go into the bathroom's showers and we wash off before heading back into the lake for some water wrestling.