On the other hand, there is plenty of water in the oceans, so it is easy for CO2 molecules close to the ocean surface to react with that water and remain there. Carbon Dioxide is very soluble in water because the gas molecules find it easier to be in water than in air. Think about carbonated drinks. When you open a can of soda, gases escape. As volcanoes continue to release Carbon Dioxide into the atmosphere, the gas will enter the oceans, because it dissolves easily in water.
Carbon dioxide is used in a variety of ways by organisms in the oceans.
They use the carbon for cell growth and to build their hard parts and shells. So as carbon dioxide enters the oceans from the atmosphere, it is removed from the water by living organisms. When they die, their remains sink to the ocean bottom and accumulate. Eventually, that material becomes the rock we call limestone. Putting the carbon dioxide in limestones gets it out of the oceans for long time periods - until the rock is transformed in some way and the carbon is released again.