getyouhome.gov

While I no longer attend the Cruise Line International meetings I still get copied on their working group meetings. Below is from one of their summaries.

US citizens  can travel on closed loop cruises from a US port without a passport if you have proper documents to identify you as a US citizen.  But if something goes wrong and you have to fly back, it is much easier to get an airplane ticket and enter the country if you have a passport

b)      The cost of a passport is expected to rise significantly, so get one before the price goes up.

Any questions about US travel requirements please see:  WWW.Getyouhome.gov.  That site will always be kept updated.

If you have any inkling that you want to travel outside the US I highly suggest you get a passport.  A passport card will provide you travel around North America and the Caribbean but the savings are not enough IMO, just get a passport. You never know when you might be fronted with the opportunity to travel and how silly will you feel if you say I can't.