Do I have to book the Cook Strait ferry
It all depends on when you are travelling and if you have a very tight schedule or not. In December and January, it is a good idea to book with one of the two companies: Bluebridge or InterIslander Especially when you prefer an early morning sail. Down site, when your trip is taking longer than planned, you need to hurry to be in time.
Want to more about Cook Strait: Wikipedia Cook Strait