Pollution, overfishing and coastal development have taken their toll on the oceans, researchers said
A school report-style assessment of the health of the world's oceans has concluded that they "could do better".
The report, based on 10 factors such as clean water, biodiversity, food provision, tourism and coastal protection, gave the oceans a global mark of 60 out of 100.
Individual scores differed greatly between coastal countries, with highly developed regions, pristine tropical islands and uninhabited regions doing best.
The UK received an average rating of 62, while the US scored 63 and Canada 70. Sierra Leone in West Africa had the poorest result, 36, while uninhabited Jarvis Island in the South Pacific was awarded the highest rating of 86.
Ecologist Dr Karen McLeod, from Oregon State University in the US, one of 30 international scientists who compiled the Ocean Health Index report, said: "When we conclude that the health of the oceans is 60 on a scale of 100, that doesn't mean we're failing.
"Instead, it shows there's room for improvement, suggests where strategic actions can make the biggest difference, and gives us a benchmark against which to evaluate progress over time. The index allows us to track what's happening to the whole of ocean health instead of just the parts."
The results, published in the journal Nature, reflect the gulf between rich and poor parts of the world. Many countries in West Africa, the Middle East, and Central America achieved low scores while higher ratings went to parts of North America, Northern Europe, Australia and Japan.
Human activities such as overfishing, coastal development and pollution have taken their toll on the oceans, said the researchers.
Lead author Dr Ben Halpern, a US ecologist from the University of California at Santa Barbara, said: "Is the score far from perfect with ample room for improvement, or more than half way to perfect with plenty of reason to applaud success? I think it's both.
"What the Index does is help us separate our gut feelings about good and bad from the measurement of what's happening."