http://www.bahamas-travel.org/bahamas-cruise-ports/
"I need a passport to cruise to the Bahamas?
I'm trying to go on a cruise CANAVERAL PORT OF BAHAMAS AND COME BACK TO DO need a passport if I am a U.S. citizen, born in the U.S.?
No, you do not need a passport for a cruise of closed circuit starting and ending at the U.S. port themselves. See website below and read the section on closed loop cruises.
Carnival Sensation video “4 nt Bahamas Cruise” ex Port Canaveral