The data rates you get on the ship are going to make using Skype and or Video calls all but impossible. Since they are using satellites fro the network connection, the delay will also kill the voice /video quality. (You will notice at least seconds of delay in video and definitely have so much delay in the voices that you will think you are talking to someone on the moon) It is like the old days when you made an overseas call, the delay was so bad that you had to say something... wait a second fro the other person to receive what you said and then get an response back. All that time waiting while you are being charge per minute charges to be connected.
You will be much better off it you have a smart phone or the ipad connected via WiFi (not through the cellular connection) and use the text messaging part.
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
NCL Dream October 2006, RCCL Mariner OTS June 2007