I went to the Port of Miami website and found this address. I guess you just address it to the ship and the person's name. At least that's how I did it back in the 80's when I kept in touch with some crew members.
Sitmar Fairwind - June 1985, Sept. 1985, June 1986
Sitmar Fairsky - June 1987
Carnival Fantasy, March 2002
Carnival Fascination - April 2004, October 2005, Dec 2006
Sovereign of the Seas - Sept. 2003, April 2005, Oct. 2005, Sept. 2006
Carnival Sensation - September 2007