Can't rate this trip highly enough. Doubtful Sound is quite different from Milford, in that it lacks the dramatic sheer cliffs, but it definitely has its own magic. It is a lot bigger than Milford Sound and the overnight cruise is definitely the way to go to experience it.
The boat was very comfortable. Our private cabin was compact (as you'd expect) but very nice. It did get a bit hot overnight but I was very impressed with the facilities.
There is a good balance of (optional) activities and downtime just watching the scenery go by. The commentary was informative and entertaining. I'm quite fascinated with the 'Catastrophic Forest' and with the way the landscape has been carved out by ancient glaciers. We also saw dolphins, seals, and an albatross (Mollymawk).
The big boat has a shallow keel, allowing it to get in close to the cliffs in some places. We had calm weather so were also able to go out in to the open ocean and run across the front of the fjiord with the sails up. Standing at the front it was easy to imagine an old-timey sailing ship and the first explorers.
We were unsure what the quality and quantity of food would be and actually packed some snacks just in case. Don't do this. The food is amazing, better than most restaurants, and it just keeps coming. I honestly can't believe they prepared all of it on board the ship!
On top of all of this, what really made the trip was the staff. They must have worked like dervishes behind the scenes to make the experience as seamless as it was, but their smiles never slipped. They were unfailingly attentive and approachable and I was amazed to see how many different jobs each individual did. Well done, crew!
My partner and I are now kind of Fiordland junkies and are planning our next trip. Our two visits so far have been sunny and I'm dying to see the area in the rain.