Yes it uses the Sprint network to access the Amazon account for downloads. But, I would assume that the Cellular at Sea service (if available on your ship) would be able to handle that "call" BUT be warned that international roaming rates would apply.. that could rack up a serious sized bill downloading books at $2 a minute plus roaming fees. Even if you don't download, does the Kindle periodically check back to the "mother ship" at amazon? If you bring the kindle, make sure you can turn off the wireless part of it (put it in airplane mode) just in case.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013