The Rainforest facility is co-ed. They give you robes, but you must wear a swimsuit because of the mixed company. it's a fairly large, open area, with the showers and rooms scattered around. None of the showers or rooms are private in this area.
I'm not sure if you can book it online, but it's simple enough to just show up at the spa desk in the morning and buy the day pass, which I think is still $16. You can come and go as you please. They only issue a certain number, so getting there early would be key if you don't/can't book it online.