The wife and I took a cruise a few years ago and I tried my damnedest to turn off anything even remotely resembling "yes, you can have a data connection". I wanted to leave my phone on in case something happened with the kids (and so I'd have a watch), I just didn't want it to DO anything other than allow incomming calls. Got home to a $200 phone bill because there was a checkbox somewhere I'd missed and cruise-ship mobile data is abhorrently expensive.

It happens.