Yes it costs and it will be at the roaming rate. Unless you really need your phone, go into settings>General>Network and turn off Data roaming (Data roaming can get high. Next turn on Airplane mode once you get to the ship but then enable wifi if you are going to buy into the ships wifi network. Most importantly, listen to music, have fun, interact with others and forget about everyone else who is not there with you!
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Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/