I found this reply by @natefish95, posted July 2021.
"For the free one, enter the gym on Deck 12, and the locker rooms are on the right. In the locker room, it's tucked around the corner, but I'm sure you can find it once in the locker room lol. It's pretty small but room for maybe about 4 people comfortably?
I highly recommend taking the spa tour on day 1. That tour influenced me to book the thermal suite. Worth the price. Always nice, quiet and relaxing, and a great way to meet people if you're traveling solo (like I usually am). "