Though passengers on some Alaska cruises that depart from and return to the same U.S. port aren't required to carry a passport as of this writing, it's always a good idea to bring one if your ship travels through Canadian waters. For one, in the unlikely event that you must fly out of a Canadian airport during your trip (a health emergency, or a problem with the ship, for example), a passport will be required to reenter the United States. Also, a passport is required for certain shore excursions, such as the popular train or bus trips from Skagway that cross the nearby U.S.-Canadian border. Further, a passport is the simplest way to prove your identity and citizenship. In other words, the trouble of obtaining one before your trip is a small inconvenience compared to the delays and possible fees that will result if you must obtain a passport in an emergency situation In any case, a driver's license is never sufficient for travel between the United States and Canada.
Though children under 16 are not required to travel with a passport, some cruise lines have different rules for minors traveling with only one adult. Parents traveling with small children should bring photocopies of their children's birth certificates to avoid any problems.
U.S. Passport Information
U.S. Department of State. 877/487–2778; travel.state.gov.