Hi DEM, welcome to cruise-chat. We didn't do the dune buggy's, we did a great 2 stop snorkel. I can't imagine it would be very bumpy, the island is pretty flat. It is a very clean island and they've built a nice terminal area with shops and a beach. It sounds like the dune buggies would be a fun excursion. If you have time, try to go on a snorkel. The water is great. We did ours through Costa. The first stop was for beautiful coral and the second stop was for colorful tropical fish. Best wishes and have a wonderful cruise!
We stopped at Grand Turk last fall. We booked our snorkling excursion through the ship, but I did look at what was available. Oasis Divers (http://www.oasisdivers.com/)does excursions - and they were who did the ship's excursion also. They were great.
Sorry, carlg, I didn't get back soon enough. We did book through Costa. I don't think it was Oasis, but could have been. Susanmaz, did they take you to the spot with the friendly barracuda? He was huge, but didn't bother a soul.
We went to Gibbs Key and fed and swam with the stingrays. We even got to try fresh conch from the bottom of the sea.... It was beautiful and way cool. I did have a problem with the feeding, though. There was a large fish who "crashed" the feeding frenzy and was pushing me to get at the conch I was trying to feed to the stingrays.... pushy little bugger...