I've had the rare (really rare) erection in a public shower, more semi-hard than anything. Between the gym that I used to go to and the pools I swim at, I've seen more than a few. At both places, you rarely see a guy wrapped in a towel, primarily naked, sauna too.
Anyway once at the pool, one of the other swimmers who's a regular, and usually showers in his swimsuit, took it off one time to shower. In no time, he was hard and trying to hide it. Musta been the reason why he showered in his suit - couldn't control it. This other dude on the other hand, gets hard quite regularly in the showers. The first time, he was in the corner (it's an open shower), the next time it happened, I said 'Dude, you're hard again!' He said he couldn't help it (yeah right). Continued showering and stayed hard, even with three of us in there. I will say, it was quite impressive and I know he knew that.
Then at the gym, I was done swimming and put my stuff in the shower (they have stalls there). I whipped off my suit and let it hang under the water to rinse off chemicals and was heading to the sink to shave. There was already a guy at the sink shaving, he had nothing on and like most of the other men around there, neither did I. When I first saw him, he was soft. After about a minute of shaving next to him, he was getting hard, we could see other in the mirror. I just smiled back him like it was ok, don't worry about it. That was it. Pretty sure I was the trigger. I'm surprised by the number of erections I've seen at this place though, for the most part the other men seem cool with it.