All cruise lines strongly recommend you get a passport, but it's not required for "closed loop" cruises in the Western Hemisphere. http://www.cruisesavvy.com/savvyguide/know_before_you_go/do_i_need_a_passport
Yes
You don't need a passport because it is a cruise
Yes, when crossing the boarders to other countries, you always need a passport.
I am not sure either!
If it involves going to another country, yes. And if your cruise lands somewhere other than Canada, Mexico, the Caribbean, or Bermuda, you'll need a passport book, not just the passport card. Source: http://travel.state.gov/passport/ppt_card/ppt_card_3926.html
Yes, you need a passport for a cruise to Canada.
Yes, you need a valid passport to cruise to Canada.
Yes, you need a passport to go on a cruise to Canada.
For travel by sea, you don't need a passport for re-entry to the United States but you do need some kind of documentation (see link below). If you are traveling by air, then yes you have to have a passport.
Basically yes, to get on AND off the cruise ship you need a passport.
Yes, you need a valid passport to go to Canada on a cruise.
If you are a U.S. citizen and the cruise leaves and returns to the same U.S. port you do not need a passport to cruise to the Bahamas.