This happened about 12 years ago. A curious 13 year old made the event even better.
I had gone back to visit my sister in the mid-west one summer about 12 years ago. I stayed in a hotel that had a big swimming pool with a couple of slides and a hot tub. I was recruited one day to watch my nephews while they swam. They brought along their cousins (a boy their age and his sisters 11 and 14)
I had a lot of fun with the kids racing down the slides and eventually my youngest nephew, who was afraid to go down the slide, decided that he would go if he could ride on my back. Well this turned into everyone wanting to ride on my back as we rocketed down the slides. I was getting aroused having the developing pre-teen and teen jumping on my back and pressing their firm young breasts into me. I used the splash into the pool and helping push them to the side as an excuse to accidentally brush their breasts, crotch and ass. I had to call it quits when my erection would not subside and I couldn't get out of the pool and go back up the ladder without it being obvious that I had a big boner.
So I moved on to the hot tub where I could still keep an eye on the kids. Well my condition wasn't going to go away and as I was the only one in the hot tub, I pulled my cock out of the side of my swim trunks leg and started masturbating while watching the female beauties as they swam, got out of the water and frolicked around. As I was rubbing myself, a young girl, looked like she was from the farm and about 13 got into the hot tub with me. I froze and put my hands to my side. I looked down and saw that she could see my cock.
Her sister and brother came to join her, I sat up so my member would slip back up the leg of my trunks and not be seen by the new arrivals, and went back to keeping an eye on the kids. When the crew that had come to the tub decided it was time to go back to the pool, the 13yr old stayed. She had positioned herself directly across from me, with her back to the pool. So I knew that she stayed hoping to get more show and I obliged. I was trying to be inconspicuous and gently raised my torso higher in the water so the view would be clearer. I rubbed faster and faster churning the water doing my best to conceal my acitons from the world outside the tub. As she stared right at my hand flying up and down, I raised high enough so that my swollen purple head was just starting out of the water and came out of the water all the way as my load shot into the air and landed back in the water. I slowly sank myself back down to the bottom with my cock still out, as the water swirled with my cum. I rested, enjoying the aftermath and then put myself away, she looked right at me and I smiled at her, she smiled back.
I pleasured myself many times after that thinking of her and imagining her thinking about me as she pleasured herself.