The problem is that when the ocean absorbs carbon dioxide, it ends up as carbonic acid which lowers the pH and this makes it harder for plankton and shellfish to form their shells.
The ocean is getting more acidic.
The
pH has already dropped from ~8.2 to ~8.1 and plankton levels are about 40% lower now than in the 1950's. The acidity varies depending on the composition of the nearby coast - largely granite land like New England and the pacific northwest have more acidic ocean water than areas that are largely limestone, like the mid-Atlantic coast. As luck would have it, colder water means better fishing, and these areas most often have granite nearby. Also, colder water can absorb more carbon dioxide.
So, this is bad for the base of the ocean food chain - which is bad for the whole ocean.