Photosynthesis started in the sea, and is still going strong there. I couldn't find exact numbers as too how much of the oxygen is produced at sea, but plants and trees are not solely responsible for the Earth's oxygen.
Conclusion: With no plants/trees, there is a source of oxygen.
This acusation is false, and I'm really surprised how you managed to come to this conclusion, photosynthises is indeed possible in the sea but by PLANTS, the light source that goes through the water casts rays on the plants which gives it energy to produce glucose, what plants use as nutrients. It is true that there is oxygen in the ocean but because of specific chemical reactions you get h2o, however because of high surface tension water creates. (having a higher positive charge tha negative) you would not be actually able to breathe the oxygen attached to the carbon.
Plants and trees are the organisms that produce oxygen, without this then there would be no renewal of oxygen...
Conclusion: plants and trees are what MADE the earth habitable by CREATING oxygen so without this how else would oxygen be made


