What are the ports of call? If any of them are not US based then yes take your passport.
Life is too short to let the ship of your dreams sail without you.
Carnival Destiny Feb. 2006
Carnival Fascination Feb. 2007
Royal Caribbean Grandeur of the Seas Feb. 2008 The DTW & MsBJ tour
Carnival Valor MsBJ and Dina Feb. 2009
Carnival Glory MsBJ and Dina Feb. 2010
Carnival Dream MsBJ and Dina Feb. 2011
If it is before June 1, a passport isn't required, but highly recommended! What would be required is proof of citizenship and proof of ID. So a government issued picture ID and a certified original copy of birth certificate and/or naturalization papers would do. Which also are exactly what you need for a passport.
Majesty of the Seas - 10/03 & 02/07 & 11/08
Enchantment of the Seas - 10/04 & 10/11
Mariner of the Seas - 10/05
Vision of the Seas - 10/06 & 09/07
Carnival Liberty - 10/07
Adventure of the Seas - 9/08
Ruby Princess - 12/09
Voyager of the Seas - 9/10
Carnival Spirit - 11/10
Oasis of the Seas - 9/13
Do yourself a favor and get a passport. So much easier getting through security at the airport. TSA will get you through quicker with it. Bob
I actually have my passport, but have not used it for international travel yet (aside from a previous cruise, but no stamps there, plus it's a Job requirement for me.) It's my father who has let his lapse, if you re-read the OP.
Mine expired last year, took less than 2 weeks to get a new one. Have your father get a new one, very easy and quick. You can even print out the application on-line and have it filled out and ready to go.