Life cannot form in the presence of oxygen
- The chemical precursors to life are not likely to form in the presence of oxygen.
- Water, H2O, contains oxygen.
- Therefore, chemical precursors to life are not likely to form in the ocean.
The problem with this argument is that the chemical properties of water and gaseous oxygen are very different. Chemical behaviour is determined by the shape of the outermost electron orbit and the number of electrons in it, not simply which elements are present. An H2O molecule has three atoms. O2, gaseous oxygen, has two atoms. Just with this simple observation, it's clear that these two chemicals are going to have differently shaped electron shells and thus different properties.
Furthermore, scientists think that the early atmosphere of Earth did not actually have large amounts of oxygen in it until after life arose, so the argument is irrelevant. For more information, see the Wikipedia articles on Abiogenesis and History of Earth.