These naturist swims have been happening for YEARS, at least since the 1980's
Strangely, so have sexual assaults. Our weekly class swim in the local swimming pool was in the 1980s, during open mixed-sex adult and child sessions. I was assaulted more than once by an older male swimmer. I was very short-sighted so friends started warning me he was heading in my direction. Couldn’t get anyone to take it seriously - the teacher laughed at me when I told her. It took several complaints by others before he was barred (and that was all that happened to him of course).
Here’s the thing - adults with ill-intent towards children tend to gravitate towards events and places where the can get access to them, especially if other adults provide it by selling tickets. One would hope attitudes towards safeguarding had moved on a bit from ‘no big deal’ since the 1980s.