I regularly workout at a local YMCA and always shower after my workout. I've gotten partially hard (thicker and longer but not hard as a rock sticking up at the ceiling) many times. But I don't stand there in the shower and wank off. Honestly, I just try to get in the shower, get cleaned up and get out of there without either attracting too much attention or ogling the other guys. Occasionally, I have seen other guys that also seem to be sporting a bit of a chubby. And just once or twice I've seen guys that looked really hard. If they weren't erect when I saw them, then I'd be amazed to see what they looked like when they were.
In the distant past, I've belonged to other health clubs (Bally's, other local brand clubs, etc). As far as I can remember, it was the same back then. I'd always shower after my workout and most of the time I would have no reaction at all. But once in awhile I'd get partially hard.
In the distant, distant past back in high school, the group showers after P.E. were mandatory. I would say that probably half of the time I got at least partially hard in the shower, along with a number of the other guys. And on any given day, the odds were pretty good that there would be one or two guys with a full hard-on, myself included.