Jetskier is right.
The only way to contact them by mail is to mail it to:
John Doe, #XXXXXXX
M/S SHIP NAME
PO Box 025589
If you do not have the employee number....SOMETIMES, if you put their full name and position on the ship...it will make it to them.
Of course, the position of Cruise Director, ACD, Social Host etc will be easier to locate.
Other positions in staff and crew get harder.