Short answer? Bring it. Long answer? It may not always be absolutely necessary.
Cruise Port
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Whether you’re sailing the Caribbean on a mega cruise liner like Carnival or taking a scenic European river cruise with Viking, you should bring your passport on a cruise. But this particular topic isn’t cut and dry.

Here’s the confusing bit: U.S. citizens on closed-loop cruises can enter and leave the country with proof of citizenship other than a passport. That includes a government-issued birth certificate or identification card. Note that social security cards and hospital-issued birth certificates don’t count.

Closed-loop cruises are itineraries that begin and end at the same U.S. port of call. Destinations on closed-loop cruises that permit passport-free travel for U.S. citizens include Mexico, Canada, the Caribbean, the Bahamas, and Bermuda.

If your itinerary included ports of call in South America, Europe, Asia, Africa, Australia—even an epic Antarctica cruise—you’ll need a passport. You may also be required to fill out additional visa paperwork.

It’s important to check with your individual cruise line about the required immigration documents. Crystal Cruises, for example, states: “All guests who travel abroad are required to carry a passport valid for six months after the end of the cruise.”

Cruise Critic, for one (and most major cruise lines), strongly recommends all passengers travel with a passport, even in the case of closed-loop sailings. In an emergency situation, you could find yourself stranded in a foreign port. With only a driver’s license, you’ll have one heck of a time getting home.

Melanie Lieberman is the Assistant Digital Editor at Travel + Leisure. Follow her on Twitter and Instagram at @melanietaryn.