The eight-year study from more than 250 scientists finds that infant sea creatures will be especially harmed.
This means the number of baby cod growing to adulthood could fall to a quarter or even a 12th of today's numbers, the researchers suggest.
The assessment comes from the BIOACID project, which is led from Germany.
A brochure summarising the main outcomes will be presented to climate negotiators at their annual meeting, which this year is taking place in Bonn in November.
The Biological Impacts of Ocean Acidification report authors say some creatures may benefit directly from the chemical changes - but even these could still be adversely affected indirectly by shifts in the whole food web.
What is more, the research shows that changes through acidification will be made worse by climate change, pollution, coastal development, over-fishing and agricultural fertilisers.
Ocean acidification is happening because as CO2 from fossil fuels dissolves in seawater, it produces carbonic acid and this lowers the pH of the water.