Seychelles and Germany have the healthiest seas of any inhabited territory, while Sierra Leone has the unhealthiest, according to a new index that says many oceans score poorly for biodiversity and as a human resource.
Topping the list with a score of 86 out of 100 was the uninhabited South Pacific territory of Jarvis Island, owned by the United States, as well as a clutch of other unpopulated Pacific Ocean islands.
The Seychelles, one of only two developing nations in the top 12, ranked fourth with a score of 73 out of 100 -- the same as that of Germany.
The index was devised by researchers in the US and Canada who measured whether the world's oceans are able to provide food and recreation while also sustaining sea life.
They examined the overall condition of 171 exclusive economic zones (EEZs) -- sea areas managed by coastal countries and stretching up to 200 nautical miles into the ocean.
The 171 EEZs represent 40 percent of the world's ocean, but yield the bulk of sea-derived food, recreation and means of livelihood.
Put together, the EEZs scored 60 out of 100, suggesting "substantial room for improvement", said a report in the journal Nature on Wednesday.
"Humans undoubtedly have substantial negative impacts on the ocean, and index scores are negatively correlated with coastal human population," it said.
Nearly half of the world's seven billion people live near the coast.
Developing countries in West Africa, the Middle East and Central America generally scored poorly, while richer nations in northern Europe, Canada, Australia and Japan had higher scores.
There were some notable exceptions, with developing country Suriname joining Seychelles in the top 12 while Poland and Singapore from the first world were ranked among the worst performers.
The lowest score of 36 went to the West African state of Sierra Leone.
The researchers measured the oceans in 10 categories including food provision, their ability to support coastal livelihoods and economies, clean water, coastal protection, artisanal fishing, carbon storage, tourism and biodiversity.
"The index is an important tool to assess where we've been and where we want to go," study co-author Benjamin Halpern, of the Center for Marine Assessment and Planning at the University of California at Santa Barbara, told AFP.
"This is the first time that we can quantitatively and directly compare and combine hugely different dimensions -- ecological, social, economic, political -- that define a healthy ocean."
He added the index only looked at how each nation managed its own EEZ, not on how they were affecting those of other countries.