Though not cheap, if you can afford it, this is a fantastic choice to get a better experience in Fiordland by staying overnight on the boat. It is a bit convoluted to get there, though all the logistics are handled for you by the company. Van ride from Te Anau to Manapouri (20 mins), then boat ride across Lake Manapouri (50 mins), then van ride from West Arm to Deep Cover (20 mins) where you finally board the Southern Secret. The scenery is great all along the way though and the drivers do some good narration to make it like a bonus tour.
Once on the boat, the crew is great. The meals were surprising good given the small kitchen they have to work with. There's options to Kayak and fish with the fish contributing to the evening's dinner. You get lunch when you first board, dinner later that evening after all the activities and then breakfast in the morning.
Cabins were nicely set up, though cramped as you would expect on a boat. The upstairs main common area was super nice with a library, games and tea/coffee and snacks at any time.