To start off, I was about 6 when this happened, and my female cousin was about 15. The adults all trusted her, and often (as they did in this story) left us alone in the pool.

The parents had gone inside for drinks and TV and stuff, so we were alone again in the pool (we were out of view of anyone except ourselves). Almost as soon as we were out of view, she swam to the jets coming out of the wall, and rubbed her pussy up to it, through her bathing suit. At the time, I didn't know what she was doing, but she looked like she was really having fun, so I swam over and stood next to her, on another jet.

IT WAS BLISS! My first orgasm, and I was underwater and not having to do anything but stand there. I managed to keep in my moans, even after 10 minutes, but then I climaxed and let out a small yell of joy, that I hid by ducking under the water.
Afterwards my cousin smiled at me and asked me to get out so we could talk in the bathroom (our changeroom).
I agreed, because I thought I owed it to her for showing me such a great masturbation technique, but was still shaking when we were getting out- and not from the cold!

In the changerooms, she took off her bathing suit and I took off mine and I was shocked to see how red and puffy my lips were! They were like twice the size of normal!
Anyway, she just said "Mmmmmmm, it's soo fun isn't it!"
I just nodded, still not fully over the fantasticaness of the orgasm.
"Was that your first orgasm?" She asked, now grinning and fingering the outline of my pussy.
That nearly sent me into climax zone again, and I grabbed her shoulder and shuddered, while she kept fingering.
After ANOTHER orgasm, we talked for like half an hour about birds and the bees, and then put our clothes on, left, and never really talked about it again until the next time we were left alone in the pool.

After that experience, I masturbated with their pool jet about every time I went over there, and still do!