Civilian ships in general constitute one of the many glaring gaps in the plastic model industry's coverage of important transporation-related subject. It actually isn't hard to compile a pretty complete list of plastic ocean liner kits. Here's an attempt:
Titanic (Revell, Academy [in several scales], Entex, Airfix [reissued Academy] and Minicraft [some being the same kit in different boxes; I can't sort them all out)
Lusitania (Entex)
Mauretania (Airfix)
United States (Revell and ITC/Glencoe)
Brasil/Argentina (Revell - long out of production)
Queen Mary (Revell)
Oriana (Revell)
Queen Mary 2 (Revell and Heller/Airfix)
Queen Elizabeth (Airfix)
Queen Elizabeth 2 (Airfix and Entex)
France (Airfix and ITC/Glenco)
Canberra (Airfix)
Then there are a couple of pretty nice Japanese liners, in the 1/700 "Waterline Series," by Aoshima and Hasegawa. I particularly remember the Hasegawa Hikawa Maru, a really nice kit that was also released in hospital ship configuration. I don't recall the names of the Aoshima liners; I think there were two or three of them, but all were essentially reboxing of the same kit. (I think.)
And Revell has released several tiny 1/1200 liners: I believe I've seen ads for a Titanic, QM2, and a cruise ship called the Aida. There may be a few more 1/1200 styrene liner kits out there.
I imagine some other Forum member will think of a few I've omitted, but those are the plastic liner kits I can remember. It's not much of a list.
Revell has demonstrated a little interest in civilian ships lately. I sure would like to see a series of 1/700 liners from the great age of the Blue Riband - to include British, American, German, French, and Italian liners. What an enormous potential there is!