Read here. Just to summarize, there is not enough fossil fuel carbon to turn the oceans acidic. Ocean "acidification" is a fool's nightmare.
"In the oceans, there is a buffering reaction between the sea floor basalts and sea water. Sea water has a local and regional variation in pH (pH 7.8 to 8.3). It should be noted that pH is a log scale and that if we are to create acid oceans, then there is not enough CO2 in fossil fuels to create oceanic acidity because most of the planet’s CO2 is locked up in rocks...When we run out of rocks on Earth or plate tectonics ceases, then we will have acid oceans...In the Precambrian, it is these reactions that rapidly responded to huge changes in climate (-40 deg C to +50 deg C), large sea level changes (+ 600m to -640m) and rapid climate shifts over a few thousand years from ‘snowball’ or ‘slushball’ Earth to very hot conditions (e.g. Neoproterozoic cap carbonates that formed in water at ~50 deg C lie directly on glacial rocks). During these times, there were rapid changes in oceanic pH and CO2 was removed from the oceans as carbonate. It is from this time onwards (750 Ma) that life started to extract huge amounts of CO2 from the oceans, life has expanded and diversified and this process continues (which is why we have low CO2 today)."