When I was in my teens my best friend and I frequently jacked off after school in his bedroom. His parents were working and not home. We used to sit side by side on the edge of his bed and look at dirty magazine pictures and jerk until we both came.
One day we went to a lake that had a public swimming area. There was a floating deck in the lake that was anchored there so it wouldn't drift. People would swim out to it and lay on it or dive off of it, etc. It was probably in about 10 feet of water.
We were both at the deck hanging onto the side of the deck as we tread water. Looking under the deck you could see across to the other side which was about 12 feet across. There were two hot girls hanging onto the other side talking. My friend suggested we jack off under the water as we looked at them. In minutes we were both holding on with one hand as our free hands jacked our dicks. The girls had no idea we were jacking off looking at them. We both shot our loads under the water and then we pulled our swimming suits back up.
Later that night ... it was a weekend night and he was sleeping over at our house, we laughed about what we had done but got hard-ons talking about it. It was late and my parents were asleep, but we locked my bedroom door for safety, took our pants off, and we jacked off again together. Actually, we ended up jacking each other off for the very first time. We never jacked each other off again, but it was huge fun and I still get hard thinking about it all these years later.