Apparently Dear Abby & Dear Ann Landers (twin sisters) did have some letters about skinnydipping.
https://sites.google.com/site/historicarchives4maleswimming/home/ii-archives-early-20th-century-after/b-syndicated-columnists
Using Newspapers.com, I was able to confirm that at the very least, the first column in that article is real:
https://www.newspapers.com/newspage/18077808/ Couldn't confirm the others, though, but I'm willing to assume that if one is real, they're all real, at least on that specific webpage. If anyone wants to do more research, feel free.
This actually kinda only cements my hunch that the Dear Abby column posted on the previous page is fake. The fake Abby waxes poetic about how grand it is that boys can and should be nude for several sentences. The real Abby is much more short and to the point, rarely using more than a few sentences, and still provides nuance, by saying that the parents should have been consulted first or by poking at the double standard of having boys be required to swim nude but not girls. She's not militantly pro-male nudity. Again, I could be wrong, I've read a very small sampling of the archive of Dear Abby advice articles from that era, but that's what it looks like to me.