Men are required to wear tight speedo swimming costumes and not the loose variety. I think it is supposed to make sure your pubes don't float off!
Let's not beat around the bush. The Speedo rule is there because some perverts used to go commando in their quite loose and shortish board shorts. They then would lay around the pool trying to flash the mums and maids.
Happened at our condo a few years ago. Our maid was laughing one evening, wiggling her little finger, and it took quite some time for her to explain the situation. Apparently she and the other maids were mightily unimpressed with the package on display. This guy was visitor to the condo and they thought a local.
When you tightly button down a population you really get some weird shit happening. Still recall my surprise upon seeing in Japan a vending machine with school girl's used nickers. no joke.