“by your logic, no one should ever even so much as touch water--evah! BTW, no one can see through other swimmers, either--they all drank too much muddy water before entering the pool!
The rule against fins are there because swimfins used for scuba/snorkling are HARD plastic, and can injure a person when kicked by a swimmer wearing them.
However, her costum is a softer and more plyable outfitting. The rule is being applied (fairly, unfairly, rightly or wrongly) to keep other people from bringing regular swimfins. Because we all know how jealous folks can get: "if she can, why not MEEEEE!?!?!" (because *you're* not nearly as pretty in that costume--be thankful we're even letting you in the pool wearing spandex over all that blubber--AND PLEASE DON'T TAKE IT OFF!!)”