Actually is is more than just filtered. They distill drinking water from the sea water and then purify the result. This manufactured water is as pure or maybe more pure than any bottled water.
Why do people bring water on board? Because they have no understanding of the above concept. We actually bring a bottle or two (big ones) so that we can refill them and have water to take off the ship in port, but only because they charge a lot for a bottle of water on the way off the ship....
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