Certain more legalistic sects of Christianity have some peculiar points of doctrine about people of both genders mingling in a body of water. Such bodies of water include bathtubs, swimming pools, and that lake at summer camp you loved to jump in. What's more, the issue is not bathing per se, as soap and sponges are not required to run afoul of this regulation; indeed, all involved parties may be fully clothed, and it is still considered an egregious sin.
My friend Will used to tell me stories about going to camp as an 8 year old and not being allowed in the lake while the girls in the camp across the lake were swimming, and visa-versa. I used to laugh at him for it, mostly because I thought he was just making it up, or maybe because he just remembered it wrong. Boy, was I wrong.
Now, I'm as puritanical as the next guy, but I've had a hard time finding scriptures to back up this rule as written. I can guess at the original intent behind it, which is sound enough, but it seems to me this may be a case of hermeneutics gone badly awry.
My first personal encounter with this doctrine was in 1993, when my then girlfriend decided to get baptised while at college. At the time, she was attending Johnson Bible College, which is affiliated with the Christian Church denomination (similar to the Church of Christ in many ways, including their denial that they are actually a denomination; the biggest difference I could find is that these guys think musical instruments are Ok in church services); the Christian Church bans mixed bathing in no uncertain terms.
The college has segregated swimming pools (men and women); no male may enter the female section of the pool facility, and no female may enter the male section of the facility. Similar policies apply to the streets upon which male and female dorms are located - the only co-ed dorm is the one where married students live. But getting back on-topic, special permission was needed from the administration on two separate issues before the baptism could happen. First, her mentor (a male prof at the college who was to perform the baptism) needed permission to do so in the campus pool. Second, we (her male friends) needed permission to enter the female swimming pool area (and we were under strict orders to not so much as touch the water) to witness the event.
She and I should have just gone down to the French Broad River with her friends and taken care of it ourselves. Granted, that would have been a bit rebellious, but she was a Methodist so (in the name of ecumenicism) I doubt they would have expelled her for it.