This is 100% true and it was very enjoyable.
I was at a water park during this past summer and there were about 15 minutes before we were going to leave. I was there with my friend and his family. I wanted to take a shower and masturbate, thinking I could look at someone while undressing then masturbate in the shower.
So I go into the changing room and start undressing. Almost everyone was naked. I saw some very nice penises and thought I had seen enough, so I went to the shower. I undressed over there, hung my towel up, and started washing myself. I did this quickly so I could maximize my time. Out of luck, the light hit a few of the reflective tiles, allowing me to see the person in the stall next to me. They were naked, and the guy had a nice cock. It was about 6 inches long, pretty thick, and cut like me. The best thing was that he was masturbating by looking out of a hole in the curtain.
The stalls were up against the wall, with two stall walls, and a curtain. I figured this was better than picturing what I had saw beforehand, So I began masturbating to him. After a few minutes, he looked down at the tile and saw me masturbating. He leaned over to make sure that I was, and I waved at him. He immediatly grabbed more lube (Which was in his suit pocket) or more soap and lathered up his penis. We watched each other for several mintues until he got down on his knees, facing me, and wanked even harder. By doing this, he would shoot his cum on the wall, and it gave me a better view of his cock.
This turned me on even more, since this was my first experience masturbating with another person. I quickly cummed, and so did he. I dried off, put my suit back on and left. I have been masturbating to this for a few months now. What makes this even more interesting is that I was in the changing room earlier, just to take a peak and check it out, and I saw him in the same shower stall. I loved it and would do it again.