This info is out of date.
The lockers on both ships have been changed to a digital system where you set your own combination. To use the locker room, you need only to walk into the locker area, choose a locker, follow the instructions to set your combination, and go to the shower. There are towels available right outside the shower area.
Note...there is a $16 charge to use the showers in the rainforest area. Showers in the locker room area are free.
If you do a spa tour on the first day, you'll know where everything is and not require any assistance. Otherwise, a CM will show you on your first time.