As carbon dioxide levels increase in the atmosphere the amount the oceans absorb also increases, leading to chemical changes that result in the oceans becoming more acidic. While the oceans are becoming more acidic, (technically, less basic) they are still on the basic side of the pH scale (basic - netural - acidic). It is highly unlikely that the oceans will ever become actual acid. However, even small changes in the acidity of the oceans can result in severe ramifications.  If they become too acidic, corals and other organisms will be unable to build new skeletons and shells and those that already exist will begin to dissolve.

The oceans absorb carbon dioxide from the atmosphere, which reacts with sea water to produce carbonic acid and also reduces the amount of available carbonate. Corals and other organisms use carbonate to produce their calcium carbonate (limestone) skeletons and shells. The acid that is produced eats away at the shells and skeletons and dissolves them. The lack of carbonate means that corals and other organisms will find it increasingly difficult to produce their skeletons and shells. It will be similar to trying to build a house while someone keeps stealing the bricks!