When I was 20, I drove my 17-year-old sister and her girlfriend out to a lake for an evening swim. There was a small, lighted beach and a small diving platform 50 yards or so out. We splashed around for a while. Then my sister went back to the beach to read a book, while her friend Patricia and I swam out to the platform, which was far enough from shore to be in semi-darkness.. It turned out the water around the platform was only about 4 feet deep, but there was no ladder for climbing on to it. I helped Patricia up by putting my hands under her butt and hoisting. She clambered aboard with her long legs and dived, or more accurately, flopped into the water. I asked if that hurt and she said no. She immediately wanted to do it again, and I hoisted her up in the same way.
Over and over she did this, with me just standing in the water serving as her human elevator. Well, between the view of her long legs, the occasional glimpse of cleavage, and above all the repeated palming of her firm little ass, I was beginning to get an erection. As her enthusiasm for this sport seemed endless, I began to think she was getting some sexual thrill out of it too, and that really stiffened my rod. Finally my sister called from the shore asking if we were going to stay out there all night. 'Pretty soon,' I called back, and then I had an inspiration. I said in a low voice to Patricia, 'I can't get out of the water now.' 'Why not?' she asked. For reply, I took her hand and placed it on the bulge in my swim trunks. They were a loose-fitting kind, and my erection was sticking straight out.
Her eyes went wide, like a deer caught in headlights, but she didn't let go. 'What are you... won't it go away?' she asked. 'Not for a long time,' I lied, 'But you could help.' I pulled down my trunks and put her hand back on my penis. 'Just hold it and move your hand back and forth,' I said. Hesitantly she began, and with a little guidance from me she was soon smoothly jacking me off. Being used to jacking off where silence was important, I didn't make any noise or show any difference in the part of me that was above water, but below water my hips were writhing with the ecstacy of it. With one final hip thrust I started to come, and come and come, strands of white stuff, some of which floated to the surface. But I don't think she saw them. Her eyes were locked on to mine and I had no idea what was going through her head. At any rate, it was quickly over. 'Thanks,' I said, 'I think its safe to go into shore now.'
A week later I suggested to my sister that the three of us go to the lake again. 'If you want to go out with Patricia, you call her. Don't use me for a go-between,' she said. But at age 20 I didn't want to get involved, and so this little erotic adventure was never repeated.