The showers have lockers. Why wouldn't they? In fact, there's a whole wall of them just opposite to the showers. Each locker with a keyhole, a coin slot and small display showing how much of the prepaid time is left on the locker. All the vacant lockers have their keys sticking out of the keyholes.
There is even a helpful instruction on how to operate the whole system. All explained in text and pictures.
A hundred yen coin gives you a locker for an hour, two hundred three hours and five hundred full day until 8 am on the next morning.

Iskandar grins.

"See? You need not worry."

He looks at Waver with a slightly mischievous grin.

"Would you like to take a shower before we go for the slides?"