Acidification is not caused by how much CO2 there is in the water. It is caused by a rapid increase in CO2 levels. We are dumping CO2 in very quickly, and so are acidifying the water.

But given time, oceans will mix down the to bottom. At the bottom it will encounter very large stores of calcium carbonate. As that dissolves, it renders the water no longer acidic. As that mixes back to the top the rest of the ocean becomes less acidic. This mixing process is estimated to take on the order of 1000 years. Therefore the long term the oceans can handle all of the CO2 we're dumping into them and much more. Increasing the long-term average CO2 of the atmosphere will not make the oceans acidic.

The problem is that this mixing process is too slow to help shellfish living near the surface today. Sure, the ocean winds up at a good ph. But it will be acidic for several centuries. And that is unprecedented. In fact there is no record of any event since the Permian-Triassic extinction over 200 million years ago that featured such rapid acidification as the oceans face today. And perhaps not even that one. We estimate that several times as much CO2 was dumped into the atmosphere as what we're releasing now, but it probably was not dumped in such a short time period. And that event wiped out an estimated 90% of all marine species.