The last time I went to a women-only session it was indeed pretty busy. I suspect it's because people like me, who don't actually need it to be women-only (couldn't care less, if you want to ogle me in my cossie it's your eyesight you're messing with - bleach helps, I'm told ), but assume it will be quieter, and so head along to those sessions specifically.

I'd say they're a bit quieter than mixed-sex sessions at a similar time, but not by much.

And yeah, I don't get it either. Not just can people in the gym and in the seating areas see you, but they make it clear they can't guarantee the lifeguards on duty won't be blokes. There may also be chaps in the mixed gender changing rooms, coming out from their swim or even changing for the gym. So your chances of encountering a bloke are still pretty high, and certainly entirely uncontrollable and unpredictable.