The turnaround could start with volunteer boaties and divers, re-seeding mussels on the barren sea floor.
Or it could come from farmers on the Hauraki Plains, planting trees and using less fertiliser to reduce nitrogen run-off into streams and rivers.
How about a network of marine reserves promising more - and bigger - fish outside the reserves?
Or fixing the septic tanks and stormwater?
Saving the Hauraki Gulf is like assembling the pieces of a jigsaw, says Tim Higham, the Hauraki Gulf Forum manager.
That the gulf needs saving from human excess is now widely accepted. The declining health of this remarkable body of water has been charted by scientists in successive State of the Gulf reports, updated every three years. Last year's report was dire, yet most people, and regulators, continue to take it for granted.
The gulf is a globally significant ecosystem on the doorstep of a major city; prized for its scenic beauty, for recreation and for fishing. Its islands and tidal flats are sanctuaries for globe-spanning migratory and threatened native birds. Its sheltered warm waters are a nursery and habitat for a vast array of fauna and flora, from micro-organisms to whales. Its economic potential to tourism and aquaculture remains both a threat and opportunity.
A year ago, the annual Hauraki Gulf Marine Park Seminar reached the astonishing (though unreported) verdict that the gulf as an ecosystem has been reduced to possibly 20 per cent of its natural abundance. That may have been a bit of a whopper, but it stemmed from the 2011 State of The Gulf report which contained a host of scientific indicators of decline. As with all science, more information is needed.
The Hauraki Gulf Forum is now confronting the task of reversing those trends and "enhancing" the gulf. At this year's annual seminar mid-week, some pieces of Higham's restoration jigsaw revealed their colours and said "pick-me". Advocates for the birds, the whales, recreational fishers, marine reserves, river and stream quality controls and mussel bed restoration made passionate and persuasive cases.
Iwi representative Paul Majurey talked of the potential gains from collective will and collaborative decision-making. "There's a lot of low-hanging fruit in terms of gains to be made." He pointed to one: "One of our pet issues is the water quality in the Piako River [on the Hauraki Plains], one of the most degraded waterways in the country."
Others, notably farmers and commercial fishers, kept their colours hidden. This will not be a quick turnaround.
The gulf's decline is a familiar environmental tale of neglectful human impacts: sediment run-off from land clearance smothering coastal habitats; nutrients from dairy farming, sewage and sediment; industrial pollutants; over-fishing and bycatch; introduced pests and weeds threatening native birds and insects; invasive marine species hitching rides on ships.
It might seem straightforward enough to target each impact except that those in the firing line duck and dive responsibility, demanding scientific proof of causal links or pointing to the economic impacts on industries, ratepayers and taxpayers.
A linked problem is the volumes of legislation, policed by agencies with sometimes conflicting interests - such as rural councils keener on economic growth than environmental preservation. The Hauraki Gulf Marine Park Act is dismissed as toothless; the forum (which includes councils, tangata whenua and representatives of the Ministers of Fisheries and Conservation) as a talkfest.
And last year, the Fisheries Minister gained power to designate areas for aquaculture regardless of local planning decisions. Already, a 300ha zone for environmentally dodgy finfish farming has been set aside midway between Waiheke and Coromandel Harbour.
The gulf sits somewhere down the pecking order of environmental priorities for government and conservationists. Its incremental decline is not as visible as the impacts of intensive farming and industry on freshwater quality. Nor is a declining ecosystem as simple a sell as the looming extinction of a native bird or Maui's dolphin.
Throw in the scientific caution about tampering with an ecosystem - tackling issues in isolation can have unforeseen consequences - and the task of preserving the gulf so our grandchildren can swim in clear water or catch a feed of snapper can seem as overwhelming as the ocean itself.
Higham is unfazed - and buoyed by growing support from interest groups. They point to the success of the Land and Water Forum in bringing together farmers, industries, scientists, iwi, environmentalists and regulatory bodies to work out ways to collaboratively manage freshwater quality and use.
Heavyweight backing for getting on with the job came at the seminar from Dr Jan Wright, the Parliamentary Commissioner for the Environment. Wright highlighted the potential of fencing off streams and riparian strip planting to combat sediment and absorb nitrogen which would otherwise get into groundwater. She encouraged participants ot to wait for scientific certainty or law changes.
Some initiatives succeed almost in isolation and point to the potential for local solutions to cut through the fog. Examples include bush restoration and pest eradication covering 620ha of private land on Great Barrier and the Muddy Feet project in the Firth of Thames - an aquatic nursery and wading bird sanctuary heavily compromised by nutrient and sediment loadings. And many farmers in the gulf catchment - which stretches as far south as Putaruru - are making great environmental strides to reduce river and stream pollution.
Other changes in this ecosystem defy scientific understanding. Seabird expert Chris Gaskin showcased the return of the New Zealand storm petrel, which "breezed in from extinction" nine years ago. From a photograph of a bird in flight carrying a stalk in its claws, scientists worked out they must be nesting north of the Coromandel - but have yet to pinpoint where. This year researchers netted and released 20 birds at sea and found 17 had brood patches.
"We need funding to nail which islands they are breeding on," says Gaskin. Though a number of critically endangered birds have been brought back from the brink with human help, he says its vital the biosecurity of pest-free gulf islands is maintained.
Marine scientists are also puzzled by another development: last year's return of big kahawai to the inner gulf. Where did they come from and are they back for good?
At one point in proceedings, Higham asked how many in the audience of 200 were doing volunteer work on gulf islands. About a quarter put up their hands. And a good number of them volunteered to help in any programme to reseed the seabed with mussels. Until dredged to destruction in the early 1960s, mussel beds carpeted swathes of the sea floor in the Firth of Thames, Tamaki Strait and Rangitoto Channel.
Niwa scientists believe they can be restored, improving water clarity and providing habitat for diverse species - rejuvenating the ecosystem from the bottom up.
The gulf forum's approach is to "open up a conversation" between the many interest groups to address the difficult policy areas, Higham says. "The Marine Park Act creates opportunities to make exceptions to the rules. This is an experiment in seeing how willing people are to engage.
"If one [sector] moves to tidy up their act, then others are likely to follow suit."
Higham's enthusiasm is catching. Last year, the Auckland Council and the Waikato Regional Council (whose gulf responsibilities were split by a line on a map) agreed to join forces to develop a marine spatial plan. An approach promoted by the Environmental Defence Society, a spatial plan could involve all interest groups in a collaborative process to identify key habitats for protection, areas for economic use, anchorages and sailing grounds prized by boaties, safeguard outstanding landscapes and swimming beaches.
But some see spatial planning as carving up the gulf for user groups and inevitably privatising parts of it. "More lines on a map," says Scott Macindoe of LegaSea, a new lobby for recreational fishers and marine research. To this sector, the long-term, bulk harvesting trawling methods used by commercial fishers in the outer gulf are the biggest threat to fish stocks - and scientists tend to agree that fishing has the biggest impact on the gulf ecosystem.
"What the Hauraki Gulf needs right now is a simple commitment from the members of the forum to increase the level of fish stocks within the park to a target of not less than 40 per cent of the unfished biomass - it's achievable, affordable and legal."
The Quota Management System allows snapper - easily the most popular stock - to be fished down to what's known as a "soft limit", currently well below 40 per cent. But the QMS relies on scientific modelling and advice and scientists are the first to admit they need more information on most of the gulf's commercial species. Even snapper, until this year, had not been scientifically surveyed since 1995, the ministry relying on catch data to predict stocks.
Latest research suggests fisheries scientists may have been over-optimistic about the recovery of snapper under the QMS since 1995 - when the stock was estimated to be as low as 10.4 per cent of the unfished biomass.
Martin Cryer, of the Ministry of Primary Industries' fisheries management directorate, told the seminar that raising the minimum limit for snapper stocks would, long-term, lead to more fish for all. But there were trade-offs: "If recreational anglers want really big fish, the long-term yield must be very much lower."
And, on past form, any short-term threat to catch limits would have the commercial sector rushing to their lawyers.
Hailed as a voice of reason in the fishing debate is marine scientist John Holdsworth, of BlueWater Marine Research. Holdsworth says the gulf attracts the most recreational use of any stretch of water in New Zealand. Catching kaimoana that's safe to eat must remain a pastime of choice. He sees potential for both recreational and commercial fishers to "fish smarter" so more fish grow to their optimum size - the recreational brigade using slow jigs and soft plastic baits less likely to be swallowed by small fish.
Holdsworth, an adviser to LegaSea who sits on the ministry's Science Working Group, supports raising the target (minimum) stock limit for snapper to 40 per cent of unfished biomass. The latest assessment model suggests snapper stocks are at 1980 levels - possibly as low as 14 or 15 per cent. "We could do better."
Can the forum be the catalyst for turning things around? At this stage it's the default option. Mike Lee, the Auckland Council's representative and a promoter of the marine park, believes a park board or similar structure is needed to advocate "without conflict" for the gulf.
"We shouldn't look to the forum to actually manage or lead the park. It's a forum for useful activities such as this," he told the seminar. "But all the [forum members] have economic conflicts of interest so there needs to be a body which will speak out for the Hauraki Gulf Marine Park. We don't have that now."
Higham doesn't believe the framework needs changing. "I think we've done a hell of a lot of work to refocus the discussion to enable people to take part in enhancement, which the act talks about.
"It's about holding some conversations that aren't happening naturally together. Through that we'll be able to address the threats to the gulf," he said yesterday.
"It doesn't need to be about opposing [forces].
"I'm still getting calls from people who want to participate in restoring the mussel beds."