The American leader was to address the first day of the Our Oceans meeting, where ministers and representatives of some 90 countries will meet with environmental experts and activists.
And one of the headlines on day one will be Obama's announcement of the 4,913-square-mile (12,725-square-kilometer) Northeast Canyons and Seamounts Marine National Monument.
This is an area off the Atlantic coast of New England with three undersea canyons deeper than the Grand Canyon and five submerged mountains, home to rare deep sea coral and whales.
Commercial fishing will be restricted in the area, where scientists have warned that warming ocean temperatures are a threat of stocks of salmon, lobster and scallops.
The new national monument follows Obama's recent expansion of the huge Papahanaumokuakea Marine National Monument off Hawaii, and several more countries are expected to declare new reserves.
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The summit is to be hosted by US Secretary of State John Kerry, who himself hails from New England and has made ocean protection a priority during his three years in office.
"During our gatherings in 2014 and 2015, nations from across the world committed to designate over six million square kilometers of ocean as marine protected areas," Kerry said.
"We will build on those achievements by announcing over 120 significant ocean conservation projects, including almost $2 billion in new pledges and commitments to protect more than two million square kilometers in new or expanded marine protected areas."
US under secretary of state for economic growth, energy and the environment Catherine Novelli told reporters 90 countries would be represented along with NGOs and the private sector.
In addition to announcing new marine reserves, the delegates will discuss scientific advances in monitoring pollution and fishing and funding for clean-up measures and protection.