Like the OP, I am really puzzled by the current fashion for people to go swimming in (non swimming) shorts or indeed even in their underwear (both genders, and equally strange in my view).

I agree that some countries like France have a very clear rule about showering (although in most countries, that really means "rinsing yourself" rather than using soap etc) which I find reassuring to an extent, but in a way even more interesting are "cultural norms". For instance, you just don't see people in Japan hitting the baths or pools without having thoroughly showered first. It is so usual that it wouldn't come to people's minds to specify it as a rule but I think that the result is great as long as you are in places patronised by Japanese people and visitors wanting to respect their ways rather than "I'll do whatever I want" types of visitors (and yes, I do think that to an extent, the choice of place will mean a strong concentration of one or the other). A number of other countries like Germany and Austria also seem to think of showering as an automatic prerequisite as it should be.

I actually use hotel swimming pools a lot when travelling, and it has happened to me to get ready to but give up just because of other I believed seemed to behave in a way that struck me as unhygienic. I also dislike swimming pools that do not force people to walk through a foot pool with cleaning solution before entering the swimming pool. That is also pretty systematic in France and a few other countries and avoids many an issue.