By absorbing carbon emissions from the atmosphere, the seas avert climate damage worth up to $222 billion (163 billion euros) every year, according to an estimate released on Thursday.
Fish catches are worth another $16 billion annually, according to the report by a non-governmental watchdog, the Global Ocean Commission, which hopes that by setting an economic price on the value of international waters, the bounty will be better managed.
The study, coinciding with World Environment Day, was released ahead of two days of ministerial-level talks in Bonn that will seek to remove roadblocks towards a new post-2020 UN climate agreement.
The ocean naturally takes in atmospheric carbon dioxide (CO2) though microscopic marine organisms at the surface, which convert the gas to carbon.
The process prevents the gas from adding to global warming although it is also making the seas more acidic, which will have an impact on many ecosystems.
"While the science of carbon sequestration in the high seas is still evolving, we estimate that nearly half a billion tonnes of carbon, the equivalent of over 1.5 billion tonnes of carbon dioxide, are captured and stored by high-seas ecosystems annually," the report said.
"Based on current estimates of the economic cost of additional carbon in the atmosphere... we find that the value of carbon storage by high-seas ecosystems ranges between $74 billion and $222 billion annually."
The report said nearly 10 million tonnes of fish are caught annually on high seas, translating into more than $16 billion in landed value.
It pointed to a hotchpotch of international laws and regulations governing the oceans, many of which were poorly enforced, or not at all. This encouraged pollution, waste and over-fishing.
"There is growing evidence that the ecosystem services provided by the high seas are of high social and economic value," said the report.
"The evidence also is clear that poor management of human activities on the high seas has eroded the natural wealth and productivity of high-seas ecosystems with negative economic and social consequences for all of us."