Basically you have the right idea. It would be best though if you got in touch with Celebrity to get them to confirm the actual pier from which the Century will leave. Having said that the Barcelona taxi driver will probably be aware of the pier number for that departure. If the departure time for the Century is about 18:00 hours then you should be able to board her at about midday or even slightly earlier. The checking in procedure with Celebrity (providing you have filled in the "Mandatory Form" on their web site is a pretty smooth and well rehersed operation. Hope this helps Neil and Ida Dow.
Celebrity cruises usually leave from Moll d’Adossat.
The large boat shaped object will give the best indicaiton of what to aim for! Depsite what the docs say you usually can board from noon onwards.
As soon as the taxi pulss up the porters will be there to take your luggage and point you in the right direction for check-in.