At the moment, US citizens are not required to have a passport to travel to Mexico and the Caribbean by sea. (They must have a passport to travel by air.)
You do, however, need a government-issued photo ID (such as a drivers license) and a certified (raised stamp) copy of your birth certificate.
At some time in the summer 2008 (per the latest ruling from Homeland Security -- this could change.... again), all US citizens will need a passport to re-enter the US by any means of travel -- sea, air and/or land.
maybe yes
On most cruises yes. But not on "Closed Loop cruises" see http://getyouhome.gov for all the passport rules!
cruises to the Caribbean and Mexico can be taken with a birth certificate and a picture ID
No. You need you passport.
Only if you are Mexican.
No; just the passport will suffice.
You will need a passport when you are taking the cruise outside of the US. The reason why is so you are in different ports that you are outside of the US.
Mexicans that hold a Mexican Passport can travel to the nations that Mexico has diplomatic relations, in some cases Mexicans may need a visa, if they are going to the USA, Canada, and others, but for instance you dont need a visa if you are Mexican to travel to Europe.
No
A passport and a visa.
No. The passport is all you need.
You'll need a US PASSPORT to return to the US.