Must do. Aside from being yet another gorgeous ferry ride New Zealand has to offer, it is also super fun. Ferry is fully stocked kitchen, ten floors, a bar, sleeping quarters if you're interested and even a small movie theater and play area for kids. This was such a nice way to travel to the south island. You're just surrounded by the beautifully glacier cut landscape that shows little if any signs of human existence. A must do.
Note:The map shows one of the ferry destinations, Wellington.