Copy and paste from Princess FAQ's.
On select U.S. roundtrip (sometimes called “closed-loop”) voyages, U.S. citizens ages 16 and above may also travel with an original or certified copy U.S. birth certificate (or U.S. Certificate of Naturalization if foreign born) presented together with a valid U.S. government-issued photo identification. U.S. and Canadian citizens ages 15 and younger may travel with a birth certificate.
Alaska roundtrip from San Francisco, Los Angeles or Seattle (roundtrip Seattle cruisetours excluded)
Canada/New England roundtrip from New York
Caribbean roundtrip from Ft. Lauderdale^, Galveston^ or New York (voyages that do not call to Martinque or Guadeloupe only)
Hawaii roundtrip from Los Angeles, San Francisco, San Diego, or Seattle
Mexico roundtrip from Los Angeles, San Francisco or San Diego