I'm feeling kind of weird writing this because I come from a very religious family. My parents are both Christians and if they caught me, I'd probably be in a lot of trouble.
My story starts when I was about eleven. I had gone to my local swimming pool with my mum and brother and was just messing around. But I got tired and decided to go over to a section that had a long seat with high powered jets to massage your back. I liked it there because it was relaxing and decided to see what it would feel like actually sitting on top of the jets rather than just leaning over it. So I sat down and scooted over. My initial reaction was to jump up because the intense pressure felt like something sharp slicing into my ass. I waited a minute then tried again, moving faster. I managed to position myself right over it, slightly in front of the hole where the jet was coming from. It felt OK at the time but then I shifted and some stray bubbles went under me and up in front, pressing against my clit and it felt better than anything ever had. At the time I knew nothing about masturbation and didn't know what was going on so I moved and went away.
A few years later, I now masturbate about once a day, and know exactly what was going on. Lately I went swimming with a small group of friends and we were hanging around in the same area, messing around with floats and splashing each other. I got tired again and pulled my float over to the seat. I knew exactly where I wanted to be and sat down, positioning myself right over the hole so the jet went straight up into my pussy. I had never felt anything so good and my legs started to shake a little so I pulled my float over my legs so I could still chat but no-one could see what I was doing. I almost climaxed within about ten seconds but thought better of it, considering the noise I would probably make, so shifted forwards, calming myself down so I was back to normal. After about a minute I shifted back again, this time so the jet went right over my clit, making me shake harder. I shifted back so it was pushing into my pussy again and slowly slid one hand down to rub my clit, still talking normally to my friends. When I couldn't stand being on the edge anymore, I gushed the jet over my clit and in seconds I came shaking from head to toe but biting my lip so I didn't make a sound. I whimpered quietly under my breath as my friends were a little way away from me but didn't make much noise.
About ten minutes later we left the pool and I don't think any of them noticed and I'm glad they didn't. But that experience was amazing, being the first time I'd done it in obvious public and it turns me on just thinking about it now.