As you will be spending much of the cruise in port, turn off the phone between stops. While in port the ship's cellular relay system is turned off and you will be using the native counties carrier. Since you are on GSM you'll be able to roam throughout all of the islands (mostly on Digicel).
You'll need to be provisioned for international voice and data roaming by Cingular. Be aware that there are reports that you can no longer have the BB unlimited International data turned on when you leave and turned off when you return. I travel enough internationally to just leave mine on but the extra ~ $20 p/month charge may not be worth it to you. If that is the case then disable all BIS email as well as BES prior to your departure.
z10 on AT&T (plus an HTC One X)