Doomsday climate models have been programmed to simulate greater ocean acidification levels as a result of increasing atmospheric CO2 from human emissions.
Simply stated, this predicted dangerous "acidification" is hypothesized to make the waters uninhabitable for most marine life.
And for this to become a reality, it means that a demonstrable and consistent decline in sea/ocean pH levels should be evident since the beginning of the industrial age.
In fact, as this study indicates, researchers are finding it exceedingly difficult to locate ocean/sea waters that exhibit a dangerous decline in pH values. In addition, as this chart shows (click on chart to enlarge), any slight change in pH from human sources is being swamped, multiple times, by natures own unpredictable wide variability.
Additional ocean-acidification and peer-reviewed articles.