It's the hot water, having a penis, the public nakedness, penises just get hard.
I'd get this from time to time as a teen. The best thing to help me was immediately dropping the water temperature. The more I got hard, the more I reduced the hot water. It takes a lot to stand under a freezing cold shower tap and stay hard. It worked every time except once.
I agree that it should be no big deal to get a natural hardon in the showers, if unintentional and the guy goes about his business, seeking no undue attention. Penises get hard, fact of life. We are adding more and more human penises to the world, already over 3 billion, and I do not believe there should be any population "control" of humans. Humans have powerful reproductive urges, which they should direct towards marriage and natural babymaking. With so many penises all around, obviously a few might be spotted once in a while, hard.
People can control their behaviors, but not always their natural erection response. And I wouldn't expect a person to reduce the water temperature. But try to act normal and ignore it, and if the one with the boner, mentally suppress it if possible.
I didn't get hard in the showers, mainly because I wasn't in the showers often enough to get that inevitable stiffie. But I did often get raging boners in class, and some guy at summer camp got really hard in the showers. But nobody said anything or seemed to care. A hard penis? Isn't that sort of natural, technically?