The first Census of Marine Life (CoML) hopes to act as a baseline of how human activity is affecting previously unexplored marine ecosystems.
The international project involved more than 2,700 researchers from 80 nations, who spent a total of 9,000 days at sea during at least 540 expeditions.
It has been described as the most comprehensive study of its kind.
"This co-operative international 21st Century voyage has systematically defined for the first time both the known and the vast unknown, unexplored ocean," said Ian Poiner, chairman of the Census Steering Committee, speaking before a conference that is being held in Central London to mark the "decade of discovery".
"All surface life depends on life inside and beneath the oceans," he added.
"Sea life provides half of our oxygen, a lot of our food and regulates the climate. While much remains unknown, including at least 750,000 undiscovered species and their roles, we are better acquainted now with our fellow travellers and their vast habitat."
The $650m (£413m) research programme, involving more that 670 institutions, set out in 2000 with the aim of answering three questions: what lived in the oceans? What does live in the oceans? What will live in the oceans?
Even after a decade of exploring the planet's marine habitats, Census scientists say it is still not possible to reliably estimate the total number of marine species.
However, the collection of millions of specimens has led to researchers identifying more than 6,000 potentially new species, of which 1,200 have been formally described.
The findings also prompted scientists to increase the estimate of known marine species from about 230,000 to almost 250,000.
The Census's 17 projects covered a diverse array of research, from improving our understanding of coral reef ecosystems to exploring mid-Atlantic ridges.
Over the past decade, scientists working on the projects have published more than 2,600 scientific papers and compiled the largest dataset of marine species.
Technological advances, such as "DNA barcoding", have also given researchers access to information that was previously unobtainable.
"Many Census technologies can soon become part of a regular ocean observing system that provides timely reporting on the health of life in the oceans," explained Professor Ron O'Dor, co-senior scientist.
Although the current programme was complete, leader of Census Studies of the Future, Professor Boris Worm, said many more challenges remained.
"The rapidly changing ocean that we are now uncovering helps us to understand ourselves," he said.
"It compels us both to continue with journeys of discovery and to make wise choices in the future."