Carnival Cruise Lines strongly recommends all of their guests travel with a valid passport. However, as of June 1, 2009, U.S. citizens embarking on a cruise that both originates and terminates at the same U.S. port (closed looped) are not required to have a passport, but will need proof of citizenship such as an original or certified copy of a birth certificate, a certificate of naturalization, a passport card, an enhanced driver's license (EDL) as well as a government-issued photo ID. Children are also required to bring proof of citizenship, and if 16 and over, a photo ID is also required. Canadian and Bermudian citizens are required to have a passport for air, land and sea travel, including all Carnival cruises.
There is a bit of "catch-22" to this which bears consideration. You can board the Miracle without a passport. However, if circumstances (weather, emergency, etc.) arise which require you to leave the vessel in one of the Caribbean ports, you might have trouble trying to return to the US by air since you won't have a passport.
OUR ADVICE IS TO GET A PASSPORT NOW. YOU WILL DEFINITELY NEED ONE IN THE FUTURE IF YOU PLAN ON TAKING ANY MORE CRUISES.
HOW TO GET A PASSPORT:
You will find a United States Passport Office located in most major Post Offices or court houses. Call your local Post Office to see if they have a Passport Office, or follow the links below for more information on local passport facilities. You will need to apply in person with a certified copy of your Birth Certificate, and 2 passport photos. Passport photos can be obtained at any photo supply shop, Walgreens, K-Mart, Target, COSTCO, Sam's Club, Kinkos, most major drug stores, etc. The current passport fee is $110 and the passport will be valid for 10 years.