So modesty doesn't apply if you go swimming or to the beach? Were 17+ year olds allowed to dress like that as well?
Indeed, I've never understood the mindset that, if there's sand on the ground or it's a swimming pool, Catholic norms of modesty no longer apply.
I think it is a mistake to look at practices from the 1950s and say "well, that was before Vatican II, so that means it must be okay." There was already a lot of worldliness at that time and Catholic culture had already taken a huge beating. In my view, the existence of swimming pools at Catholic schools may
be an indication of some backsliding, although if mixed bathing were not allowed at those pools, many of the concerns are mitigated. Also, they would be private, and private bathing segregated by sex can't be compared to public and/or mixed bathing.
It is my understanding that the Church has always condemned mixed bathing. Spain is a country that has a lot of beaches, but at least one of the prelates there, Cardinal Pla y Daniel, Archbishop of Toledo, stated in 1959: "A special danger to morals is represented by public bathing at beaches... Mixed bathing between men and women, which is nearly always a proximate occasion of sin and a scandal, must be avoided."
Apparently the bishops of Spain together published in 1958 a document entitled "Norms of Christian Decency". I've been unable to find an English transcript of this document, and only found reference to it in some news articles talking about some "extremist" Catholic/Carlist group that was calling for segregated beaches and used this document to back up their position (cf. http://www.dailymail.co.uk/travel/travel_news/article-3154546/Extreme-Catholic-group-Pedro-Men-ndez-Avil-s-urges-beaches-segregated-sex-northern-Spain-sins-flesh-avoided.html
The oft-quoted January 12, 1930, instruction of the Sacred Congregation of the Council, does not differentiate between whether there is sand on the ground or not.
I think it is worth recalling the grave scandal that was occasioned when bathing dresses started appearing in the mid-late 1800s and women would go as deep as ankle-deep water at the time. It shows you how far we've come, and I think it gives an indication of what attitudes were like for centuries.
I've studied a fair bit of Medieval life and culture and I've never found references to public bathing. There were lots of bath houses, but these were segregated and, again, it's private. Wealthier people bathed in private. My impression is that on the whole, the culture of bathing was to get in and out relatively quickly, to get clean.