I think the wrong question is being asked. It shouldn't be 'Why can't you take photos at a swimming pool?', but rather, 'Why would you want to take photos of children you do not know, in particular, when they are in a state of minimal dress?'
I wouldn't be happy if people were taking photos of specifically me without me knowing. I mean, if you want my photo, why would you do it surreptitiously, without asking? By not doing it above board, it does make you wonder if there is something less than wholesome about why the photos being taken. Why would it be any different for children?
I don't worry so much about photoshopping etc, but I do worry about somebody targetting my children. It's a limited number of people go to swimming classes at the same time every week. Kids often attend in school uniforms. Names would be overheard. It'd be very easy to pick up kids names, parents names, know what car someone drove, have a good idea where they live based on school uniform. Normally this is of no concern, but then you start wondering, why does this person also seem to need a photo of my child. Again, it's just not quite right.
I also think it has to do with respect. When children are at swimming classes, they are learning. Would you mind if a complete stranger walked into school and decided to photograph just your child, or if a parent decided they needed to get some photos of their kids at their desk at school? That wouldn't be right either. So why is it okay in a pool?