Each ocean zone contains an estimated 33,000 known forms of life, ranging from algae and single-celled protozoa to whales and sea birds.
The Mediterranean was also listed as a hotspot for different kinds of species by Census of Marine Life (CoML) scientists who surveyed 25 key marine regions.
Waters around popular holiday resorts in southern Europe were in the top five of the biodiversity league table, along with oceans off China and the Gulf of Mexico.
However the Mediterranean was also one of the areas where biodiversity was most threatened.
On average, the number of known species per region totalled 10,750, ranging from 2,600 to 33,000. But for every marine species of all kinds known to science, experts estimate that at least four are yet to be discovered.
Fish made up only 12% of the marine species identified in the round-up, described in a series of papers in the online journal Public Library of Science (PLoS) ONE.
Crustaceans such as crabs, lobsters and prawns were the most abundant forms of life, making up around a fifth of species in any given region.
The next most plentiful were molluscs which include squid, octopus, shellfish, snails and slugs. They made up 17% of a region's species population.
Plant and animal micro-organisms, such as algae and protozoa, each accounted for 10% of marine biodiversity, and segmented worms 7%.
Much of the remainder was made up of other non-vertebrate animal species including sea anemones, corals, jellyfish, starfish, sponges and sea urchins.
A category listed as "other vertebrates" that made up just 2% of the total contained some of the best-known marine animals, including whales, sea lions, seals, walruses, turtles and sea birds.
The "roll call" was published in the run-up to the long-awaited final report from the Census of Marine Life in October.
Almost 400 scientists from more than 80 nations are involved in the project, which aims to provide the most accurate assessment possible of the diversity, distribution and abundance of life in the oceans.
Dr Mark Costello, from the Leigh Marine Laboratory at the University of Auckland, New Zealand, lead author of the latest research, said: "This inventory was urgently needed for two reasons. First, dwindling expertise in taxonomy impairs society's ability to discover and describe new species. And secondly, marine species have suffered major declines - in some cases 90 per cent losses - due to human activities and may be heading for extinction, as happened to many species on land."
Biologist and author Dr Nancy Knowlton, from the Smithsonian Institution in Washington DC, US, who heads the Census coral reef project, said: "At the end of the Census of Marine Life, most ocean organisms still remain nameless and their numbers unknown. This is not an admission of failure. The ocean is simply so vast that, after 10 years of hard work, we still have only snapshots, though sometimes detailed, of what the sea contains. But it is an important and impressive start."
She added: "The sea today is in trouble. Its citizens have no vote in any national or international body, but they are suffering and need to be heard. Much has changed just in the few decades that I have spent on and under the sea, but it remains a wondrous and enriching place, and with care it can become even more so."
Scientists found that many species appeared in more than one region. The most highly travelled "cosmopolitan" species lay at both ends of the the evolutionary scale, and included algae and protozoa as well as sea birds and mammals.
The "Manylight Viperfish" (Chauliodus sloani), found in more than a quarter of the world's marine waters, was described as the "everyman" of the deep ocean.
The Mediterranean had the largest number of invasive species of any region, with more than 600 immigrant varieties - 4% of its total. Most had arrived from the Red Sea via the Suez Canal.
Along with other "enclosed" sea areas, such as the Gulf of Mexico, Baltic and the Caribbean, the Mediterranean also had some of the most threatened biodiversity.
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