Never saw, or had any in school showers I can remember. Or in the communal showers in boot camp. But I remember one incident in boot camp in the navy after lights out. There were 40 boys in our barracks room. All 15 or 16, and with all the metal beds being end to end along the walls, it wasn't easy to stroke one off without everyone in the row being aware someone was doing it.
We had young divisional officer who like to give us surprise "inspections" He would come in shortly after lights out and make everyone stand on their lockers and drop their pajama bottoms. He would then come along with his flashlight to check for crabs, apparently. ( yeah right ) If he found a lads equipment particularly interesting, he would lift up the kid's package with his swagger stick and get a closer look.
The guy next to me was quite well hung and invariably have the remains of an erection slowly subsiding, or not. You could almost see the officer's eyes light up and he would pay particular attention to the kid's bits with his stick. We would swear you could see him salivate. lol.
Ours was the only division that weren't allowed to wear swim trunks during our swim classes. He always insisted in races too, and would make sure he stood behind the boys as he made sure they did the "on your marks ,set, go" routine before they dove in the pool. Making sure he got a good view of several young bottoms as the bent over to dive.
Would probably never get away with it these days, but most of us just laughed at it. I am sure some of the younger, shyer boys were probably disturbed by it.