You'd have to send it to the home port of the Carnival Legend which currently is:
South East 10th Avenue
Fort Lauderdale, FL
However, you have to label your post card right or it won't ever reach it's final destination.
First line: crews first and last name AND crew number (if you dont have their crew number, you can put their position instead)
Second Line: ship name and department of employee.
Third Line: ships home port address (listed above)