FWIW, on our last cruise (Wonder to Alaska) we used 1 large and 1 medium paid package over 7 days, and we were using a laptop every day for work, plus ipads and phones. So you can use less than a gig a day. But boy is it hard to turn off every background data service. You have to, just offhand:
- Suspend Dropbox, Google Drive, OneDrive, iCloud if on a Mac, updates for your anti-virus, updates for Flash, Java, Google Chrome, Firefox, and any other auto-updating apps you have
- Turn off automatic email downloading if you use an email app rather than webmail
- On iPad & iPhone, turn off iCloud sync (both directions), turn off automatic synchronization of everything (apps, music, updates, etc.)
Even then I don't think you would be turning everything off. On the laptop, there were still background processes moving data around, and I used some debugging tools to figure out a few of them, but several were services I couldn't find any clean way to turn off, so ultimately I signed in whenever I wanted to use the computer and signed out when I was done, but even then while I was working there was a fair amount of data transfer.
On the Windows Phone, there's a global "restrict background data" feature you can use when on a metered connection. That was helpful. They could use that feature on laptops and iOS devices. But of course there are very few situations where wifi is metered like it is on a cruise, so I understand that it's a low priority.
The bottom line is, I'm sure the person who wrote the story about the Wonder believed that he turned off all background data, but he just may have missed one or two apps or background services that happened to pick that time to download a major update. Any one service can use a massive amount of bandwidth, and it takes a bunch of debugging and analysis to track down what's using the data.
And of course it's always possible that the data provider is cooking the books. I'm just saying Occam's razor suggests otherwise. If Disney found out their network provider was cheating guests, being dropped like a hot potato would be the least of their worries.