Ocean acidification is reduction of Ph in the ocean during an extended period of time. Usually when a Ph is decreased it’s at the cost of the chemical balance of the oceanic ecosystem, which could create conditions that eat minerals that are used by marine life to build their shells and skeletons ( the chemical is calcium carbonate in case your wondering.). If these minerals are gone then the organisms would lack energy to grow important muscles in the body, even if some organisms can adapt to survive the overall health can be impacted. 

Marine pollution occurs, when littering is  done on the ocean. When it occurs trash can be mistaken for food by wildlife, and plastic attracts and gathers other pollutants in the seawater, posing a contamination risk to underwater marine life that will probably eat it. 

(Ocean acidification is common in beaches, while ocean pollution does also take place in the beach, it can also be implied to freshwater as well.)