Booooo !!! That was a bad pun chuck... but actually not that far from the truth.
They do use salt... they draw in salt water from the ocean, filter it a bit and then they use they same method to sanitize the water as many home pools do... they run the water through an electroysis grid that generates chlorine from the salt already in the water. This way the ship doesn't have to carry the chemicals around. If done in the correct amounts, you won't smell the chorline as the amount of chorline is only high enough to kill the nasties...
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, Celebrity Silhouette 2013