Did it! Booked through the cruise which is the only way I could find to do it. It was a blast! I can highly recommend it, but get your crunches going a few weeks before-hand if possible. My abs are SORE!
zip-line is good in Belize you do the zip line and Cave tubing together in there company you do it with i Belize Travel Adventure day have website with nice photo of the tours www.belizetraveladventure.bravehost .com
We did it and we had a blast, but I did not go thru the cruise line as they went to a smaller zip line place for more money. We got a contact thru the orphanage there for a guy that picked all of us up (17 of us) and charged us $20 per a person to drive us around wherever we wanted to go all day. While the adults did the zip line, again my favorite shore excursion now, even over the stingrays in Grand Cayman, he brought some of the adults with the younger kids to the Iguana Farm. If you look up on line the orphanage there in Roatan you can email them to get the contact info for someone to help you, and also consider bringing something for the orphanage, they depend on us cruisers.
We did a combination of both the Zip Line and Cave Tubing in Belize through Action Boys Belize. Carnival doesn't offer this combination for the Belize Port. This combination is a blast! The Cave Tubing part of the tour is given almost free. You can also add a tour of the Belize Zoo free of cost.
I would recomend them highly. Ask to have Donald Ramirez as your tour guide. He is warehouse of knowledge.