Any data usage while on the cruise is charged at .02 per KB used. Which works out to be about $20.48 per MB. A typical BBM message will use a few kilobytes. So each time you send/receive a BBM it's costing you a few pennies.
If you want to freely use your data while in Mexico or on the cruise ship you can add the unlimited global plan for $64.99/month. You can contact the global support team @ 908-559-4899 to add the data plan if you want. If you call from your VZW phone there is no roaming charge to call this number.