At Labadee (Royal's Island Paradise), it is a ride (of sorts). Imagine a wire line running from the top of a higher hill running down to the water, or some other landing spot. There is a sort of seat hanging from the wire, which you then ride down to the landing site. I also think there is a tree-top Zip-Line ride in Jamacia that runs through the top of the rain forrest.
If you go on the web site for your chosen cruise, and pan down to the cruise itenary, click on the Blue Highlighted Port stops, and a short list of things to do at each port are listed. As you get closer to your sail date, all the shore options will be listed for you to select.
It is a cable suspended between trees or mountains that you are connected to. You wear a harness with hooks, gloves, and a helmet. Then you zip across the cable. Here is supposedly the world's fastest.