My parents have a beach house in New Jersey. Two summers ago I was on the beach around four pm and there was a couple making out about 20 feet from me. I was sitting in a sand chair, which was one of those really close to the ground. They were really going at it, he was slipping his hand in her bathing suit and she was doing it to him. He had a big hard-on and she was really squirming.
Well, all of this really turned me on, so I put a towel on my lap and started to rub the head of my penis, that was now sticking out the top of my bathing suit. After about five minutes I shot my load. I used the towel to clean up and went into the ocean for a swim to wash off.
When I got back they were just laying there, but I knew she had made him cum because his suit was wet, right in his crotch. Too bad I didn't stay a little longer to see it happen.