We were asked to leave our local Council run pool today after a lifeguard ascertained that my 1 year old wearing just a swim nappy was a girl. His explanation was that it 'wasn't fair' and the duty manager came out with the most ridiculous reasoning. It's not disallowed but probably sensible that she wears a one piece just in case there are any paedophiles taking photographs 
how would we feel.
If she'd have been in her pirate nappy instead of a flowery one no one would have bothered.
The pool was busy with parents watching swimming lessons so it was really quite embarrassing and I couldn't actually leave because my older DD was in a lesson.
There is a reason for wearing just a swim nappy, because it is a washable one it doesn't really fit under a swimsuit that well and the two fabrics catch each other when wet.