You cannot get (as far as I know and I searched quite a lot before we went to the Caribbean in feb) a sim for multiple Caribbean islands, you'd have to register one for Barbados, one for st lucia etc etc as most are on different networks. Far better to walk off and use free wifi in any of the areas around the town. I did this with my new ipad and it was fine and fast and great for FaceTime rather than calling home and paying for the call.
If you connect your I-pad on the ship it will cost heavily regardless of your airtime supplier because it goes thro the ship's satellite gear, and often connections are slow or unreliable. ............... much better to use your I-pad ashore, where many bars etc offer free Wi-Fi - watch out for the "free Wi-Fi" signs. Just ask the proprietor for his access code. ............ of course you can draft your-emails o the ship without connecting. ........ Chris.