Normally, simply turning off roaming is the solution. However, if you have an international plan like I do, you need to have risking enabled in order to use the International plan. The problem is that you can rack up roaming fees when the phone connects to the ship's own cellular tower, so I turn off cellular data when I board the ship and turn it back on when in port.
As others have suggested, call your carrier. Mine just instituted an option where you can punch in a code to your phone to disable data roaming on any not included networks, so now I don't have to worry. And if there's an emergency, I can disable it.
