There are immense quantities of certain creatures in the ocean that take CO2, emit O2, and eventually die and sink to the bottom of the ocean, thus building deep layers of sediments and sequestering enormous amounts of carbon.
They've been doing this for almost 2 billion years: phytoplankton account for about half of all photosynthetic activity on Earth (thereby consuming CO2); 50-85% of the oxygen in the atmosphere is produced via phytoplankton photosynthesis. There have been past periods with at least 10x the current CO2 level, yet all of these oceanic organisms have thrived through it all.
[added] The massive accumulation of dead plankton in sea floors for millions of years is the primary origin story of oil and coal. Clearly oceans are a very large carbon sink.