I would regard it as perfectly normal and natural. I was slightly taken aback when a female German colleague went topless, but that's the local culture so you take it in your stride, and hopefully if you do whatever you are comfortable with, people will respect your choices.

Sorry if this seems like a daft question but why would your supervisor's presense in particular put you off? Is it their superior status, some power their authority gives them that you are uncomfortable about, is it fear that you might do something embarrassing or something that could be used against you later?

I'd say just go with the flow. I remember being uncomfortable around my superiors when we went to a music do, but in the end I did share the dance floor with them, even though they were, well, a different caste so to say, as well as a different generation and so on, but afterwards it was all good; nobody remarked on how unusual it all had been or how many boundaries had been crossed etc. It had just happened and that was all.

@user78727 As an example for point 1 and for your edit, I usually go to the swimming pool once a week with some coworkers, and there is frequently also a PhD student. Generally, my advice about this kind of situations is... behave normally ;-)

My own PhD supervisor regularly invited his grad students and others from the department to a beginning of the school year pool party at his house. No one seemed to object, or think it was in any way inappropriate.

I go to the University swimming pool and I have met undergrad students half my age. I agree with @MassimoOrtolano in taking it normally

In brief I would answer ok. Still, your question might be linked to many things, tradition, culture, religion, interpersonal skills. I never had troubles on gathering with friends and colleagues during my phd. Later on, in another group, I was basically unwilling to participate in a graduation toast, for instance. And you see, in some place it can be disturbing for a old prof to join a younger group such as his supervised staff, dressing just shorts. Not the contrary. Therefore I suggest you to relax. Again, an answer is difficult without knowing you, your boss, the colleagues and so on...

news.bbc.co.uk/1/hi/magazine/7915369.stm

Just go, but not dressed for swimming. If anyone asks, make up a white lie, e.g. "I've been somewhat under-the-weather and don't want to go into the cold water" or "I pulled a muscle" etc.

I've experienced the gender reverse: A male professor at the beach with ~20 females undergraduates. And I was quite uncomfortable.

Thanks for bringing that up @JosephO'Rourke and Alchimista. This was actually one aspect I was implying when asking the question. I think it is interesting that you are the first ones who address the issue that it might feel uncomfortable for anyone else in the group. I think this is a crucial point since it also affects how you should behave yourself. When someone notices that others might feel uncomfortable has a huge impact on group interactions, in my opinion.

"It's no secret that sexual objectification of women..." <- Sorry, this is polarized as hell. "it sounds a bit naïve to neglect that partial nudity in a professional relationship would not play any role at all" <- So me and my ex-colleagues were naive? Co-workers going together in sauna in places like Germany, Russia, Finland are naive? The weight of this problem of yours is as big as you make it to be. Besides, it looks to me that you already chose not to go.

You're right, naive is not the right term here. It has probably a million reasons why people say professional bodies don't play a role and I'm in no position to judge this. I'll edit it.

@JosephO'Rourke why were you unconfortable? Were they making advances? Were they trespassing personal boundaries? In any other case, I'm sorry my friend, but it's you who had a wrong reaction. It was not the wrong situation to be in.

You know you can't take electronics with you into the bath..? I'm sure you can figure out the rest.

@Noldor130884 So because some people are okay with it, it's not okay not to be okay with it?

@Noldor130884 So me and my ex-colleagues were naive That seems probable.

@Miguel and I know plenty of colleagues who pick when they swim to avoid exactly that.

@user78727, as your second EDIT is longer than the rest of the question altogether, just a hint on this problem: it is often the case that people suggest the "X-Y" problem (i.e., you ask X but people think you really want to/should solve problem Y). Sometimes this is a very valid point, but in your specific question, I would say it is totally uncalled-for. I find your original question both sufficiently informative (i.e., I know exactly what you are asking) and I see zero need to make it about you as a person, or your relationships. So I would not bother too much about it. ...(cont.)

@user78727 ... (cont). In the same vein, I find the answer you accepted not appropriate either (nor the highest-voted one). They say "of course it is acceptable". It is absolutely not "of course" and there is a lot more to say about the topic than just point blank denying that there's an issue at all. Same for all the other answers which are slightly or directly blaming you for asking the question at all, calling you insecure or whatever. In the end, I'd suggest that this particular SE question just has gone hopelessly wrong, but not due to you asking anything wrong.

@AnoE One reason why I chose this answer was certainly because it was actually an answer after all :) I completely agree with you that it is not simply "of course", but I did ask that question in the first place. So it wouldn't make sense just to wait until someone will finally state what I considered to be the "right answer" from the beginning. I actually wanted to know if it is considered as "normal" in such a context and even if I don't regard it as "natural" it is significant that so many seem to be upset by just questioning it. So I received the answer that at least the social setting...

@user78727: no need to explain, I just wanted to give you a heads-up that there's no need for you to be feeling so defensive. You surely managed to hit the hornet's nest here. ;)

@AnoE ...is not uncommon, which I aimed to find out with my first question. How people actually feel and think about it is not answered but was also not part of my question. I do also agree with you that this question went wrong to the extent that many commenters just refused to address the actual topic. Thanks for pointing that out again!

@AnoE The comments and some of the answers seem to indicate that the second edit is very much necessary. I have to admit, my own first, knee-jerk, reaction was to frown at the question as odd and unnecessary. But both edits make strong arguments for why the question is a good one.

I voted to close because the second question depends entirely on personal factors.

Is it ok to go naked sauna with all your colleagues (of the same sex) as a social program? If not, the organizers of one conference probably broke the policy :-)