Photosynthesis in the earth's forest produces about half of the oxygen in the atmosphere. photosynthesis in the earth's oceans produces the other half of the oxygen in the atmosphere. why do you think so much photosynthesis occurs in the oceans ?
(think about what type of organisms perform photosynthesis)

Photosynthesis occurs in oceans due to presence of phytoplanktons in the surface water.

Explanation:

Phytoplanktons are the phototrophic organisms that are found in surface water of the oceans. These are very tiny organisms and are present in large number in ocean surface water.

The phytoplankton release oxygen during the process of photosynthesis. Presence of a large number of these organisms in oceans leads to the production of as much as half of the oxygen present in the atmosphere.