I just returned from the most AMAZING cruise ever on the Mariner which is not the same ship but.......maybe this info. may help. Not sure about the Explorer and all but on the Mariner you did not need to pay to use the steam room and sauna or whirlpool at the gym/spa. You can use it as often as you like. I didn't figure that out until mid week and would have used it more right from the beginning if I'd known. So it may be the same on the Explorer if they have the steam room and such.
The Explorer did have a sauna and a steam room in the locker rooms when I was on it about 2 years ago. All free. I don't remember when it opened but it did close around 8:30p which I thought was early compared to times on Celebrity ships.
All the shipshape centers on the RCCL ships have steam rooms and saunas in the locker rooms. There's no charge for them. If you want to get a locker key, just go to the spa desk and give them your seapass and they'll give you a key to the locker for the time you are there.
They're open from the time the gym is open until the time the gym closes.
Yes, on the Explorer of the Seas, there is a sauna (womens/mens), exercise equipments, massage and a beauty salon inside the Day Spa and Fitness center on dect 11 and 12. The sauna and exercise euipment are free of charge. When you get onboard, make sure you attend the Spa tour as there are a lot of free give aways. hope that helps.