At any public location where it is acceptable to be nude, if it's your first visit being publicly nude, erections are understandable, we've all been.
I'm a 24 year old male and this is my experience at a korean spa. Unfortunately it is not coed nudity so its not really a nudist spa or anything because male and female nude areas . One gasped as I reached a full erection.
Nudist Erection - Ever wonder what nudists think about penile erections? Find out about naturist etiquette and how naturists address erection.