Assuming you are going on a Carnival Cruise you should contact your carrier. Service is availiable on all Carnival ships at international roaming rates. Verizon for example charged $2.49 a minute international roaming charges while at sea, as of September 2007. The carrier should also be able to give you a per minute international roaming rate from Mexico while in port. You may have to add an international plan to your phone.
One word of advise though I'm not sure about Mexico you may find it cheaper to use an international calling card (pre paid or otherwise) then your cell phone. You should compare rates.
RCI Monarch of The Seas 1998
Carnival Miracle 9/24/07
Upcoming Fascination 11/5/09