For a cruise to the Mexican Riviera I would not recommend taking medication for malaria. You could end up sick and not enjoy your trip. Put on bug spray before you do the interior things like the zip line. We have done Peru, which was all interior and the Belize zip line, loved it, too short a time, and just used our bug spray.
You can check http://www.cdc.gov/travel/regionalmalaria/camerica.htm
for information about it.
I would recommend to anyone traveling to anywhere in the Caribbean to get Hepititis A and B shots. You can protect yourself from malaria easily with bug spray but the bigger problem could be Hepititis. It takes 3 shots over several months and if you travel on all of these great trips to the Caribbean at anytime it is worth it to protect yourself. Eating and drinking in port and such can leave you vulnerable. I am far from an alarmist. I belive it is like putting your kids in car seats and fastening your seat belt. Sort of defensive traveling. For that matter all it takes is eating out in the US where someone with Hepititis works and handles food to get it. Talk to your Dr. but don't be surprised if he doesn't really know about what is required for travel to foreign ports. He probably never has enough time to get away. We also got Yellow Fever vaccinations when we went to Peru. Read some of the sites about Malaria and such on the internet but don't get crazy. It is pretty safe traveling to ports where the cruise ships go.