Well, I too cruise a lot.. here is the information straight from the largest cruise line in the World
CRUISE TRAVEL
U.S. Citizens
Carnival highly recommends all guests travel with a passport. Although a passport is not required for U.S. citizens taking cruises that begin and end in the same or a different U.S. port, travelling with a passport may help to expedite your CBP clearance upon return to the U.S. Additionally, passports are required in the event you need to fly from the U.S. to a foreign port should you miss your scheduled port of embarkation, or need to fly back to the U.S. for emergency reasons.
The Western Hemisphere Travel Initiative (WHTI) allows U.S. citizens (including children) sailing on cruises that begin and end in the same U.S. port to travel with one of the WHTI compliant documents, listed below. Additionally, the same document requirements apply when taking a cruise that begins and ends in a different U.S. port.
•
Valid U.S. Passport
• Passport Card
• Original or suitable quality copy of a Birth Certificate (Issued by a government agency: state/county/city) and a government issued photo ID if 16 years of age and older.
• Original Certificate of Naturalization
• Trusted Traveler Program Membership Card, e.g., Nexus Card, Sentri Card or Fast Card
• Enhanced Tribal Card
• A Consular Report of Birth Abroad
• Enhanced Driver’s License (EDL) – only acceptable at land and sea ports of entry. It cannot be used for air travel outside the U.S. The word ‘Enhanced’ must appear on the driver’s license. The following five states currently issue an EDL: Michigan, Minnesota, New York, Vermont and Washington. -- So, NOT TRUE for everyone
Copies of U.S. passports and naturalization papers, hospital and baptismal certificates are not WHTI compliant documents, therefore, are NOT acceptable.
Birth certificates from Puerto Rico issued prior to July 1, 2010 are not valid forms of proof of citizenship that is accepted by U.S. Customs and Border Protection. Guests from Puerto Rico either need to present a WHTI-compliant document or a government-issued photo identification with a validated birth certificate issued after July 1, 2010.
*If you need to obtain a passport quickly, Carnival recommends that you contact VisaCentral at
www.visacentral.com/carnival or by phone: 1-866-788-1100. Callers should mention that they are sailing on Carnival to receive discounted service fees.
http://www.carnival.com/core/faq.aspx?icid=CC_Footer_88#q-543535