E.T.
If she is wearing them in the water they need to be shoes that will stay on her feet and have a rubber type bottom to prevent slipping. That will be important especially if she will be wearing them in a lake or creek. If she is going to be doing actual swimming and not just playing in the water they need to fit pretty snug. Otherwise they will just fall off of her feet and float away. If she is just going to wear them in the locker room and on the pool deck the Croc type of shoe will be fine.