We did the Rain Forest tour a few years ago. Just wear casual clothing. Mostly, the tour was in the bus, with a few stops where you can leave the bus but stay near it to view waterfalls etc. You might take a cap or hat, but other than that, I wouldn't worry about it. Have a great time, and when you return let us know what you thought of the cruise, and the rainforest tour.
Have a wonderful first cruise! It won't be your last !
"A truly happy person is one who can enjoy the scenery on a detour."