The five-day meeting is the first bid by the United Nations to address the toughest problems facing our oceans, from coral bleaching and plastic pollution to overfishing and rising seas due to climate change.
"We must put aside short-term national gain, to prevent long-term global catastrophe," Guterres told the gathering at the UN General Assembly.
"Conserving our oceans and using them sustainably is preserving life itself."
President Donald Trump's decision to withdraw the United States from the Paris climate accord cast a long shadow over the gathering aimed at agreeing on a strategy to reverse the oceans' decline.
From the UN podium, Bolivian President Evo Morales took a swipe at the United States, saying the withdrawal from the Paris accord was "denying science, turning your back on multilateralism and attempting to deny a future to upcoming generations."
These actions are "the main threat to Mother Earth and to life itself," Morales said.
UN member-states are working on a "call to action" to be signed by countries who commit to taking steps to clean up the oceans and work to preserve what is arguably the Earth's most important resource.
Targets include protecting at least 10 percent of coastal and marine environments by 2020, reducing ocean pollution and strengthening ways to fight illegal, unreported and unregulated fishing.
In his remarks, Guterres raised alarm over the state of the world's oceans.
Pollution, overfishing and climate change are causing severe damage, he said.
He cited a recent study that showed that plastic could outweigh fish in the seas by 2050 unless action is taken.
Rising sea levels threaten entire countries, fisheries are collapsing in some places and coastal ecosystems are under unsustainable levels of stress from industry, fishing, mining, shipping and tourism, Guterres said.
The UN chief said the first step to turn the tide is "endiing the artificial dichotomy between economic demands and the health of our seas."
"The conservation and sustainable use of marine resources are two sides of the same coin," he said.
Guterres called for concrete steps, from expanding marine protected areas, to the management of fisheries, reducing pollution and cleaning up plastic waste.
Previous global meetings on problems facing the oceans have taken place, but the five-day conference is the first one being held under the auspices of the United Nations.
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