This is according to the last info I could find on Google....
But a passport is a good thing to have....and will be necessary soon...
If you're a US citizen and planning to cruise to Alaska on a cruise that stops in Canada before summer 2008, you do not need a passport. You can also cruise to the Caribbean (except Barbados), Mexico and the Bahamas without a passport.
For any of the small ship cruises that sail only in Alaska, you do not need a passport.
The Department of Homeland security expects to announce the new date for requiring passports for all travel outside the US later this spring.