And, depending on where you plan on traveling, a valid passport plus a visa may be required.

For ALL cruises, no matter the destinations, you will be required to have acceptable proof of citizenship.

From now until the end of 2005, all US Citizens .. Acceptable forms are a birth certificate with a raised seal plus a drivers' license, OR a valid passport.  Some countries will even require a passport and/or a visa.

Check out this official government site: http://travel.state.gov/passport_services.html. It lists every country, whether or not U.S. citizens need passports and/or visas.  This information may change at any moment for requirements in documentation. 

Just a note .. you may be asked to turn over your documentation and airline tickets to cruise line officials at the start of the cruise. You will receive your documents back. 

All Non-USA citizens must have valid passports, alien registration cards, and the requisite visas when boarding any cruise ship or aircraft departing from and/or returning to American soil. Non-citizens also need to present an ongoing or return ticket for an airline or cruise ship as proof that you intend to remain on local shores only for a brief stay.

And, all Non US citizens will need to verify with their consulate if  they will be required to present further documentation to leave their country and visit any other countries.  There are countries that some citizens will not be able to travel.  Always check first.  World travel changes all the time, and if you do not have proper documentation, you will be refused passage on cruise ships and airlines.

After you have booked your cruise and made your deposit, you may go online to a participating cruise line and pre-register.  And, you can verify your cruise reservation directly online with the cruise line.  Even look at shore excursions that are offered on your sailing and purchase excursions online.  Plus lots of information about your cruise in online.