As of June 1st, all Canadians will need passports to enter the U.S. by land or water. This is in addition to air travel which has required a passport since January 23, 2007. This initiative is required by the Western Hemisphere Travel Initiative (WHTI), a U.S. law requiring 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. Document requirements for children do vary somewhat from adult requirements.
Additional details are available from the Canada Border Services Agency.