It really depends on what your clients needs are. There are a couple of good shuttle companies (Orange comes to mind) and there is always the cruise ship transport. They problem they may have is staying in Orlando. Most of the shuttles pick up from the airport. While I am sure you can have them pick up at a hotel, I am not sure if there would be an additional charge.
We have just rented cars (which can be more economical if you have more than 2 people in your party) You can do one way rentals and drop the car off at the rental car store in Cocoa Beach. Each rental car company has a shuttle from the store to the port and back so you can easily rent in both directions.
I'll let others chime in on shuttle company recommendations as I have not used anyone other than Orange and that was local to Orlando.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007