The ability to pre-book Wi-Fi packages is currently only available for the Carnival Vista and Carnival Fascination. Carnival has announced plans are in place to have this feature available fleetwide by mid-summer 2017.
Please note, the pre-sale requires guests to purchase a Wi-Fi package that lasts for the entire cruise vacation. However, for guests that do not want coverage for the entire cruise, daily packages will still be available for booking onboard. Carnival currently offers 3 Wi-Fi packages that offer different levels of coverage. More information about Wi-Fi packages can be found on Carnival's website.
To read the press release where I got this information: Click Here For Carnival's Press Release (Opens In New Window)