Through a number of different processes oceans can be a significant carbon sink. An increase in the carbon content of water does acidify it so this is not something we are interested in, but in healthy seas there is a great deal of plankton and that plankton does absorb carbon dioxide. It is furthermore believed that some of this carbon, locked away buy plankton, may actually make it to the ocean floor to be locked away long term at least in part. Indeed it is thought that carbon dioxide concentrations would be at least 50% higher wear it not for this part of the carbon cycle.
However plankton does not do well in water that we have substantially degraded. If we wish the oceans to work the way that they do naturally we need to stop dumping all our waste into the waters. If we wish for oceans to play their part as a carbon sink the food chain in these Waters must be intact. This includes not only the plankton but the krill and the larger fish, as well as the Marine mammals such as whales.
Whales themselves are a huge carbon sink, it is thought that the combined carbon storage of all large baleen in Wales before whaling destroy their populations would have added up to about 910 million tons. It is thought that allowing well numbers to recover would remove 160000 tons of carbon from each year- and likely to increase over time as the population grows.
Another good carbon sink also happens to be very good for Coastal communities. Mangrove forests and coral reefs pull carbon out of the air and and significantly reduce storm surges and large tides making it easier for people near the coast cope.