I think you will be ok with 2 liter bottles but I am not sure about 2 - 2liter bottles. As stated early we bring one bottle. Enough to make sure we get to the first port. Then you can buy additional soda in port and carry it bake on the ship without a problem. In port the only thing they hold would be liquor. Then you don't have to worry about lugging bottles of water and soda AND you luggage onto the ship on embarkation day.
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