I'm having trouble with this. At some point I added a phone case to the outfit, but I've since removed that. In the locker room it says you can't leave because you're not wearing a bottom (for the femme uniform.) A once piece swimsuit and the cap.
Ah yes, that's a confusing bit I keep meaning to sort out. If you're trying to get to a swim meet, you can also join it from the pool, so just go that way after you're in your swimsuit.