Easiest is to have
Airplane mode - on
WiFi - on
She must authorize her phone for the ship's WiFi. The Internet manager can help her with that if she can't do it herself. She does not have to log to use her minutes just for using iMessage. Just needs to be connected to ship's WiFi.
As long as Airplane mode is ON there is no risk for roaming, but unless you have a roaming plan turn off roaming. I'm not in USA and don't know what plans you have but I suspect that international plan does not cover roaming at sea. I would suggest roaming OFF the whole time for a non-tech-savvy person. However, if you want her to be able to use roaming while ashore, make sure to tell her to turn on Airplane mode when she comes back to the ship. You don¨t have to be tech-savvy to do that. You "just" have to remember to do it! (NOT as easy as it sounds says I, tech-savvy but forgetful!)