Instead of getting a passport, why not spend that passport fee on piña coladas instead?
Because things have changed and you must now have a passport to fly out of and back into the United States from almost everywhere, including those countries previously exempted. Plus, as of June 1, 2009, the Dept of Homeland Security's Western Hemisphere Travel Initiative (WHTI) requires "all citizens of the United States, Canada, Mexico, and Bermuda to have a passport or other accepted document that establishes the bearer’s identity and nationality to enter or depart the United States from within the Western Hemisphere." That means by air or by sea.
However, your cruise may be exempt. U.S. citizens on closed-loop cruises (cruises that begin and end at the same port in the U.S.) will be able to enter or depart the country with proof of citizenship, such as a birth certificate and government-issued photo ID. Essentially, that hasn't changed. However, you might need that passport even if your cruise line doesn't require it for your sailing. Find out why and get the information you need to know about your personal identification requirements. Take Cruise Diva's advice and Identify Yourself With A Passport.