I have only used a rental car out on CUN (4 1/2 Hrs. on good safe paved roads) so I do not know the cost and time with a taxi, again it's a good road and 90 Km/HR is not out of line down from Mahahal. I would contact the XTC dive center and see if something could be set up ahead of time. If there are enough divers in the group they might be able to pick you up in a van or pre arrange transportation for you. The big question would be the time in port of your ship. The best dive I did was La Poza and La Chimenea and the two tank dive, with surface interval takes 4 hrs., from the dock back to the dock. With everything set up in advance including paperwork I think it would take about 7 hrs. to do the trip, minimum. Most cruise lines I have heard of spend a minimum time in port so they can get their captives back to the bar and Casino so I think the time in port is the limiting factor. Again the best source of info would be to E-mail XTC (reserve@xtcdivecenter.com) and see what they recommend. They are a great and very professional group, off the grid and at the end of the world (or at least Mexico).