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.
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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.
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 "ending the artificial dichotomy between economic demands and the health of our seas."