To get to the South Island we took the ferry which runs from Wellington to Picton. We returned our car at the ferry terminal in Wellington. Although you can take cars on the ferry the rental companies do not wish you to do so. To me it was odd in that you park your car in a certain area of the lot and drop you keys in a return box. I felt some degree of anxiety about the car company saying there were issues with the car but it works out fine. On the other end in Picton the car rental agencies are right outside the terminal doors.
The ferry terminal in Wellington is not near anything of interest so once you are there you are stuck. They did have a little snack counter but not much else.
The ferry ride was nice and smooth. I stayed on deck most of the time to catch the views and to take pictures. We had opted for the upgrade and so had plush seats. The upgrade also includes food and drinks. Since I do not drink all that much and did not wish to drink that early in the day we did not eat and drink the extra cost.