I have done this in the past - and it was really easy.
All you have to do is supply proof that you own when you book the cruise, and that you still own it when you take the cruise.
Here is current information I have found - it appears they intend to end this program someday...
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Booking Deadline: 2/28/13
Ships/Saildates: Applicable on sailings through 7/31/13
$ 50 on board credit per stateroom ($25pp) on any cruise of 6 days or less
$100 on board credit per stateroom ($50pp) on any cruise of 7 - 13 days
$250 on board credit per stateroom ($125pp) on any cruise of 14 days or longer
The following restrictions and limitations apply:
This benefit is available to shareholders holding a minimum of 100 shares of Carnival Corporation or Carnival plc.
Employees, travel agents cruising at travel agent rates, tour conductors or anyone cruising on a reduced rate or complimentary basis are excluded from this offer – CPGO/CPGI/CMGM/CCAC/No T/A Rates/SA’s/CCLM/CCSA (net rates).
This benefit is not transferable and cannot be used for casino credits/charges and gratuities charged to your onboard account.
Only one onboard credit per shareholder-occupied stateroom.
Certain restrictions apply.
Applications to receive these benefits (OBC) should be made at least two weeks prior to cruise departure date.
After the reservation has been made, the shareholder must provide their name, reservation number, ship and sailing date, along with proof of ownership of Carnival Corporation or Carnival pic shares (example: photocopy of shareholder proxy card; shares certificate or current brokerage or nominee statement) at the time of deposit to Guest Services Admin. This information can be faxed or mailed at the time of deposit to:
Carnival Cruise Lines
ATT: Guest Services Admin Supervisor
Guest Admin, Mailstop MSGA-457s
3655 N.W. 87 Avenue
Miami, FL 33178-2428
Fax Number: 305 406-6477
Mary Ruiz Online Community Specialist Carnival Cruise Lines Staff
Note - you should also take proof of ownership with you onto the cruise so you can prove it to the front office if the credit is not on your account already.