On the Voyager of the Seas those who can take their own luggage off the ship can do so at 7:30am.
No matter what time you get off the ship as soon as you walk out the door you are at the mercy of US Immigration and they can make the process as long or short as they please.
The cruise ship companies have no control over this process.
February 1998 - HAL Westerdam E. Caribbean
January 2000 - Carnival Ecstasy - Mexican Riviera
February 2002 - Costa Atlantica - W. Caribbean
February 2004 - Grand Princess - W. Caribbean
March 2007 - RCI Voyager of the Seas - E. Caribbean
May 2007 - RCI Voyager of the Seas - Western Mediterranean (Mrs. E. & daughter only - I had a medical emergency)
October 2007 - RCI Adventure of the Seas - Southern Caribbean