Although the WHTI (Western Hemisphere Travel Initiative) passport rules started in mid-2009 requiring U.S. travelers flying overseas to own passports, some cruise passengers can sill travel without them. Although a cheaper Passport Card or a microchip-enhanced Driver's License, available in some states including New York, is still acceptable on "loop" cruises starting and ending in the U.S., the Carnival Splendor's recent power failure and stranding at sea serves as a warning to those without passports.
Had Splendor's 3,000 + passengers been disembarked at a foreign port beyond driving distance to the U.S., instead of being towed to San Diego, they would have needed a passport under WHTI State Department rules to fly back to the U.S. Those without passports could face airport hassles and lengthy delays. The warning for non-passport owners is clear. Buy a passport before you cruise, even from a U.S. port.