My advice - Get a Passport
1) Proof of Citizenship
Beginning January 31, 2008, all adult (age 19 and above) travelers will be required to present proof of citizenship, such as a birth certificate, and proof of identity, such as a driver’s license, when entering the United States through land and sea ports of entry.
This change is in addition to the following which is already in effect:
2) Passport Requirements
All travelers including citizens of the United States, Canada, and Bermuda are now required to present a valid passport when entering the United States at any airport as of January 23, 2007. This includes children of any age.
"Do I need a Passport to go on a cruise?" is probably the most frequently asked question by clients looking at a cruise vacation. My advice is that I'd highly recommend that they obtain a passport (even if less restricitve rules are in effect - such as the first bullet above).
Here are some examples of why you should have a passport:
a) You are traveling on a cruise after Jan. 23, 2008, and have valid proof of citizenship (other than a passport) as outlined in the first bullet above. You, or a member of your family becomes ill and you need to fly home. Your proof of citizenship (such as drivers license and birth certificate) is now insufficient, since the more restrictive airline rules take effect.
b) You miss an airline connection, or don't get back to the ship in time and you need to fly to the next port of call to join / rejoin the cruise. Unless the next port of call is a US territory, you will most likely need a passport to enter the country. (there are some exceptions to this, so if you are a gambler, you could roll the dice and hope you won't need a passport).
c) A no brainer situation is one where you are traveling to a country or countries that have passport (and possibly visa) requirement for entry and exit. Check with your travel agent or the country's embassy if you are unclear of the requirements.
U.S. Dept of State - Passports (information about requirements and how to obtain a passport)
Passport and Visa Services (passport & visa expeditors)
U.S. Embassies (Websites of U.S. Embassies, Consulates, and Diplomatic Missions)
Centers for Disease Control and Prevention (advice about any health advisories here and abroad)
Traveler's Health (CDC Travel specific topics)
Flight Related DVT (Flight-related Deep Vein Thrombosis -Economy Class Syndrome)
Transportation Security Administration (TSA website - latest regulations and resources)
KNOW BEFORE YOU GO is my suggestion to you. Consult with your travel agent, cruise line, and air carrier to be sure you understand all the rules that govern your trip (including unforseen circumstances such as the examples above).