In sunlit surface waters, tiny marine plants called phytoplankton use the carbon dioxide, C02, to grow. (news.mongabay.com)
Outside of coral reefs, other marine ecosystems also benefit from lower pH. For example diatoms now account for 40 % of the world's ocean primary productivity and flourish in upwelling waters that bring abundant CO2 and DIC from the ocean's dark depths into sunlit surface regions. (landscapesandcycles.net)
When CO2 first invades sunlit surface waters, it indeed dissolves into 3 forms of inorganic carbon (DIC) and lowers pH (DIC is discussed in How Gaia and Coral Reefs Regulate Ocean pH). (landscapesandcycles.net)