DRF is wonderful. I love it. I have gone to Jamaica several times, and I make a point to add this to my list of things to do.
1. Bring a water camera. The tour guide will stop along the way for you to take pictures. You don't want to ruin your good camera.
2. Don't be afraid to get your hair wet. You will go under the fall.
3.. DO, DO...bring your own water shoes. You can get a cheap pair at Target or Walmart for less than $5. They will try to rent you some, but I prefer my own. The rocks are slippery and you need something on your feet. If you don't have water shoes, bring old tennis shoes. When you complete the climb you have to walk a distance to get back to your bus the ground is very hot to walk on, and you will pass through a straw market with many vendors trying to sell you things.
4. Before you leave the cabin, wear your swimsuit under your clothes, so all you have to do is slip your clothes off.
5. Bring a towel
6. If you want to shop, bring money, so you might want to get one of those waterproof carriers that you wear around your neck
If I recall, they have lockers, if you don't want to leave things on the bus, if you do not plan on shopping, you might want to bring a few bucks. (I forgot, you will have to carry money to pay for the cruise), so go to a place that sells luggage and you should be able to find one of the waterproof holders. They come in handy for the beach also.
Many women wear t-shirts over their swimsuits.
Enjoy. It's a lot of fun.