Explanation

The global oxygen cycle is dominated by marine photosynthesis, not terrestrial ecosystems, which is a common conceptual trap.

Statement I is incorrect. Although rainforests are highly productive, they do not produce more oxygen than oceans. A large part of the oxygen they generate is also consumed internally through respiration and decomposition, resulting in a limited net contribution.

Statement II is correct. Marine phytoplankton and photosynthetic bacteria contribute roughly 50% of global oxygen production. These microscopic organisms form the base of oceanic food chains and are the primary drivers of atmospheric oxygen generation.

Statement III is incorrect. The amount of dissolved oxygen in water is far lower than oxygen in air. Even well-oxygenated surface water contains only a small fraction compared to the ~21% oxygen in the atmosphere.

Thus, only Statement II is correct.