There are two ferry companies in New Zealand servicing the Cook Strait route, to go from the North Island to the South Island or vice versa: Bluebridge and Interislander. They both are great at what they do and will get you to the other island.
Which one you book is up to you, if you prefer to book the ferry yourself. We prefer to work with and therefore will always book the ferry crossing via Bluebridge. So your Spaceships Ferry Voucher will always be turned into a booking on a Bluebridge ferry.