The first 50 MB are free but only the first day.
To clarify: you have to sign up on the first day to get the 50MB free offer, but assuming you do, you have the entire cruise to use it.
Another thing that seems to gobble up data is Facebook on a smartphone. This can be partially mitigated by turning off the photo and video settings. One thing that's particularly bad is the feature where it starts playing a video even if you don't click on it.