This is a GREAT excursion!!! My DH and I took it while on our last cruise. You start the excursion by boating over to Water Island via a large pontoon boat. While on the boat, one of the tour guides gives a narration of the history of Water Island, and other important facts/figures for your trip. NOTE: You will be traveling in a large group of 30+, comprised of passengers from the Magic, Princess, and Costa ships. Once you arrive, you get into two open-air jalopy-type jeeps, and are taken up to the top of the island, where the bikes are stationed. Once there, you are given a quick how-to of mountain-bike riding, as well as various other safety tips (like in St. Thomas, they drive on the left side of the road!) and are fitted for your helmet and bike. Then, you're off. There are several bike-tour guides that escort you, as well as the jalopy (in case you need to stop). You ride for about 5-10 minutes, then you stop and various points around the island (great vantage points) where the tour guide will talk about the historical significance of that particular spot, etc. The riding is fairly "rough" in that you are actually riding in dirt/sand/rocks, etc. After riding for about 1-1 1/2 hours, you are given the option of concluding your tour and taking a dip at Honey Moon Breach, or continuing on for another 45 minutes or so. I choose to do so (DH stayed at the beach). WOW! This was hands-down, the hardest part of the entire excursion (but well-worth it). We rode downhill a very LOOONG way to an old bunker, then, had to ride ALLLLL the way back up (over a muddy and rocky trail). I was so tired when we were finished, but very proud that I had challenged myself. We then joined all the others back at the beach for swim before heading back (via jalopy then pontoon) to the ship.
It was a GREAT tour, and I highly recommend it. The guides are great, and make sure that you have a safe, fun, and informative ride. In addition, they provide you with water throughout the whole trip. We had people of all ages and body types on the tour and they were OK. The guides are very helpful, do there was no one who felt left out. I am fairly physically fit, and was definitely challenged. BUT--I then felt better about eating all those extra desserts all week. We are definitely taking this excursion again the next time we are on the Magic.
Have fun!