Question

why we cant break down the ocean water to produce oxygen like plants did during photosynthesis

Answer 1

We cannot break it cause it's very complex

Answer 2

The process of separation of the molecules of hydrogen and oxygen from water (H2O) in plants takes place during photosynthesis, i.e. in the presence of the sun's light energy.

Hence, as we humans lack chlorophyll and cannot trap the required light energy, we cannot split water into molecules oh hydrogen and oxygen.