I hope you have a great cruise, mgm1249. Yes when using a smartphone on a cruise ship, you would be using their tower and subject to their rates. Here's a great link: http://vz.to/17KseUf
In this link, you can see the rates and dialing instructions and settings in our international trip planner. You just have to choose what Princess Cruise Line you are boarding to see the rates.
Please keep in mind you would need an international plan if you were getting off the ship and using your device. We would be happy to help if you need any further information. Just tell us your device, if you will use your device off the ship, and what Princess ship you will be on.