Little has been done to protect marine life since the 1992 Rio Earth Summit, conservation scientists conclude.
On pledges to protect key habitat and restrict the size of fishing fleets, they say progress has been "pitiful".
Their analysis is carried in the journal Science and is being discussed during final preparations for the Rio+20 summit, which opens next week.
Conservationists were delighted by Australia's move to set up the world's largest network of marine reserves.
But globally, the picture is bleak, they say.