Less than 1% of the world's oceans have been given protected status, according to a major survey.
Governments have committed to a target of protecting 10% by 2012, which the authors of the new report say there is no chance of meeting.
Protecting ecologically important areas can help fish stocks to regenerate, and benefit the tourism industry.
The survey was led by The Nature Conservancy (TNC) and is published in the journal Conservation Letters.
"For those of us working in the issue full-time it's not a surprise, we've known all along that marine protection is lagging behind what's happening on land, but it's nice to have it pinned down," said TNC's Mark Spalding.
"It's depressing that we've still got so far to go, but there are points of hope," he told BBC News.
Coastal concentration
Four years ago, signatories to the UN's biodiversity convention - which includes almost every country - pledged to protect at least 10% of the oceans in a way that makes sense ecologically.
Protecting them does not mean banning activities such as fishing or shipping completely, but making sure they are carried out sustainably.
All of the areas currently protected fall into countries' Exclusive Economic Zones, and the majority are along coasts, the study finds.
Even so, only about 4% of coastal waters are protected.
Countries diverge widely in how much protection they have mandated; around the Mediterranean, less than 2% of the coastline is protected.
In the developing world, Dr Spalding cites Guinea-Bissau as a country that has had invested in protection, particularly in the Bijagos Archipelago, which is home to a community of hippos dwelling along its mangrove coast, as well as more conventional marine species.
Palau, Indonesia, Micronesia and several Caribbean states are also making significant progress, he said.
About 12% of the Earth's land surface has been put under protection.
Richard.Black-INTERNET@bbc.co.uk
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