Fortunately, dissolution of carbonate skeletons of existing organisms is not an issue...at least not yet. The apparent problem is that the normal life processes of several important marine organisms are being disrupted by an acidic water environment that has not been found in earth's oceans for hundreds of thousands of years. The acidity is attributed to abnormal atmospheric carbon dioxide concentrations.
The most striking and accessible coverage of the new consensus report on this issue, from National Science Foundation, the U.S. Geological Survey, and the National Oceanic and Atmospheric Administration, is found here, in a "Science Now" article .
There is a paradox to this issue. Marine plankton and coral, through normal metabolic processes, sequester dissolved carbon dioxide, forming carbonate minerals that slowly fall to the bottom of the ocean. In this manner, limestone (like your calcium pill) was formed in ocean sediments over millions of years. When dissolved carbon dioxide becomes too high (and pH too low), however, normal metabolic processes of these organisms are affected adversely, leading to the scientists' concern that plankton productivity would be reduced. By further increasing acidification, we risk locking this negative feedback loop into full open position, after which climate change would be accelerated, proportionate to the reduction of the ocean's carbon-sink function. This function is significant, and the surface area of the ocean is much greater than that of tropical forests that also sequester carbon.
Conversely, if plankton growth is stimulated before an ecosystem crash occurs, the C02 "sink" capacity of the ocean would be increased, and carbon dioxide acidification stabilized or reversed. Just such an approach has been proposed as a serious venture over the deepwater zones of the world's oceans. TreeHugger has posted about it here and here.
'Pico of the Caribbean' sez: "go read about me mate, or I'll unbuckle your swash".