I know exactly what you mean, Mustang. I'm nine years older than you and still haven't grown out of it.
I usually don't shower at a gym unless it's got private cubicles, and at the pool I tend to wear my speedos.
It's not that I get hard all the time, it's just that I'm never sure when it will happen. And if a hot guy walks in, it's game over. I guess you don't have that problem though.
As for what I'd think if I saw a 19-year old with a hard-on in the showers -- I'd be impressed that he was relaxed enough about himself to let it happen, but I'd probably assume he was liking something he was seeing.