This site should answer your questions
http://travel.state.gov/travel/cbpmc/cbpmc_2223.html
Here is a partial quote from that side
"JANUARY 31, 2008
U.S. and Canadian citizens will need to present either a WHTI-compliant document, or a government-issued photo ID, such as a drivers license, plus proof of citizenship, such as a birth certificate. DHS also proposes to begin alternative procedures for U.S. and Canadian children at that time.
LATER
At a later date, to be determined, the departments will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have either a U.S. passport; a U.S. passport card; a trusted traveler card such as NEXUS, FAST, or SENTRI; a valid Merchant Mariner Document (MMD) when traveling in conjunction with official maritime business; or a valid U.S. Military identification card when traveling on official orders."
To me "later" means that at anytime they can say "as of xxxx everyone will need a passport when traveling on cruises"
I feel it is just easier to have the passport especially since this is a longer cruise. If you there is an emergency and you have to fly home from one of the ports having a passport will make getting back into the U.S. and being able to fly out of a foreign port much much easier
The other reason is what if something happens to someone while in port your drivers license and birth certificate will not help you at that point