Here is a cut and paste from another board ...it seems you can buy passes for the day or entire cruise to the Rainforest ...
Pros: A great place to just relax and unwind
Cons: Didn't spend enough time there!
As soon as we boarded the Mediterranean cruise, I made sure to get access to the Tropical Rainforest throughout our 11 nights on board. Unfortunately I didn't get to use it nearly as much as I hoped, so perhaps I didn't get my money's worth out of it, but everytime I went, it was exceptionally relaxing and I came out of there feeling very refreshed.
My favourites are the tiled loungers, just divine to sit on them, although I learnt my lesson - make sure you put a towel down. Those things can leave marks on your back if you don't! Another favourite was the gentle sauna and the fog showers, much more powerful than the ones in your room. The steam room was too hot for me, so I didn't spend much time in there, but I loved all the other facilities in there.
You can buy daily passes or passes good for your whole cruise and for me, this is one of the best parts of the cruise. It's just an opportunity to relax and unwind completely.