It's perfectly quiet and still out on the water, though, and the only other vessels we see all day are the fishing boats moored on the wharf.
Before we complain about the weather, the clouds part to reveal the brightest blue and the skipper tells us this area is known for having four seasons in one hour, never mind one day.
The tannin-coloured tides are high under a super-moon as we cruise past the misty mountains. We're told animals that normally like deep seas are also happy here in the darker-coloured water.
Nature doesn't disappoint as a pod of bottlenose dolphins come into view. They're larger and darker-skinned than the dolphins in the Bay of Islands and much shyer - they swim briefly in the boat's wake, behaving more like wise old men than the frisky puppies their Northland cousins resemble.
After another coach ride, we take a bigger boat to the entrance of Doubtful Sound, knocked around by ocean breezes as the boat bounces in the big swell.
It's worth the queasiness to get up close to the sea lions, draped like giant leeches on the rocks.
Nearby, albatrosses skim the sea like gliders and we're told great white and mako sharks sometimes circle the rocky home looking for seal for supper. This news gets the kids excitedly surveying the sea for super-sized sharks.
Fiordland National Park, a World Heritage Area, is the largest of the 14 national parks in New Zealand at 1.2 million hectares. Rocks here are said to date back 450 million years.
When Captain Cook visited this area in a strong wind, he observed the direction the trees were growing and decided it would be "doubtful" his vessel would get out again if he sailed further into the sound, so he named it Doubtful Harbour.
We say goodbye to the sea lions and cruise back to base, but not before we're taken to the most breathtaking scenery of the tour and told to head out on deck as the engines are turned off.
We experience what it might have been like in these parts in the early days, reminded that the deep-water landscape was sculpted by glaciers retreating and advancing.
Doubtful Sound bottlenosed dolphins, larger than their North Island cousins, escort the boat like wise old men.
It's a relief to have the mobile phone games stop their beeping and pinging (we noticed one man on the boat watching a film on his iPad as the stunning scenery cruised past him unnoticed) and the only sound is the slush of water dribbling down vertiginous cliff faces.
I half expect a moa to appear out of the mist, or at least Bear Grylls.
On the coach ride back to Lake Manapouri, we're given the chance to go deep underground to view Manapouri Power Station, which is said to have saved the lake after plans to flood it for power generation came about in the 1950s, sparking the iconic "Save Manapouri" protests in the 1970s.
The driver repeats her call for people to hop off if they suffer from claustrophobia. After all the winding mountain roads, I'm quite glad my five-year-old daughter makes a dash for the door and we're able to stretch our legs, walking back to the visitors' centre before the next part of the trip.
It's a relief, but it means we miss out on seeing what is underneath this impressive feat of engineering.
On the walls in the visitor centre, I'm surprised to see words from my great-great-grandfather Peter who, in a 1904 report into the hydro-electric potential of Lake Manapouri, had suggested - more than half a century before the "Save Manapouri" controversy - that "the present beauty of the lake is worth preserving to the fullest extent".
It's strange to see his writing and imagine that long-ago ancestor standing in this spot, and to learn for the first time about his respect for the environment.
We rejoin the bus passengers and as we head home on the last boat, the mountain scene opens out to reveal a full palette of colour. We cross a lake often described as "the loveliest of them all".
Not only is it undeniably beautiful, it's also a lake with a lesson.
QUEEN FOR THE NIGHT
• The Real Journeys tours were all a cut above, with modern coaches and boats and friendly, professional service. We took a Doubtful Sound Wilderness Cruise but there are also overnight sea kayaking and four-day trips to the Sound.
• Stay at the Heritage Queenstown - at a perfect distance from the centre of town for a stroll around the lake, which has plenty of flat stones that seem to skim for miles across pristine water. Afterwards, soak in the hot tub or swim in the warm pool from indoor to outdoor. In the morning, children dine for free in the hotel restaurant's buffet. The views from the viewing deck at the hotel are sublime.
Last month, Heritage Queenstown's executive chef, Adrian Lowrey, launched a summer menu in Mackenzies Restaurant & Bar. A la carte options include manuka-smoked Ora king salmon with sauce gribiche served with watercress and perla potato salad and pan-seared Canterbury duck breast, brioche-coated duck liver parfait, carrot and orange coulis with wilted kale.
The Heritage is at 91 Fernhill Rd, Queenstown, ph (03) 450 1500.
Danielle Wright was a guest of Real Journeys and Heritage Hotels.
