The only Caribbean country that now requires arriving U.S. citizens to have a passport is Barbados. As for all other locations in this hemisphere, only an original birth certificate and a photo ID are required. This will change in 2006, however, as regards travel to the Caribbean.
Beginning 1/1/2006, American citizens will be required by the U.S. government to possess a passport for travel to and from the Caribbean. In 2007, that requirement will extend to Mexico and Canada as well. However, I should point out that many Caribbean nations are protesting this announced U.S. policy change as favoring Canada and Mexico over the Caribbean during the 2006 travel period. It remains to be seen how the U.S. will respond to these allegations.
The US has responded by making ALL US citizens show a passport when they re enter the country. I spend a lot more time outside of the US than I do back home in the States and I have found that in a lot of countries it is the only acceptable form of ID if you want to rent a car, stay in a top hotel, or heaven forbid, need the assistance of either the local police or US consulate. The way things are going...it may be the only acceptable form of ID anywhere so I would just go ahead and get one. They are good for 12 years.
"Leaving on NCL Sun May3 do all passengers need a passport and proof of identity or do Canadian/US citizens only need a passport (I hope) for boarding at check-in?"
A PASSPORT is sufficient - you do not need any additional ID.
It only gets complicated if you don't have a passport.