You do if you have booked a "cruise only". However if you have booked a complete package including flights, then the cruise line organises this. This may be a dedicated cruise charter flight or scheduled flight depending on the cruise. Whoever you book with should give you the details.
If you book cruise only transatlantic on QM2, will Cunard or an agent then be able to help you to get 1-way flights separately?
Yes either Cunard or agent can book flights. If booked with Cunard, this will probably include transfers between cruise port and airport. Alternatively you can book the flights yourself, depending on which way is most cost effectively. The advantage of booking a package with Cunard, which can be done through an agent, is that if anything goes wrong with the flight, then Cunard are responsible for sorting it out. Cunard also do hotel packages if you want a few days in New York. You need to look at the costs of the alternate ways and then decide...Brian.