But really it depends on how much water there is, because like you said "Water is H2O (hydrogen and oxygen)." depending on how much the volume of the water is there will be more oxygen. But oxygen isn't what's all in the air that we breathe. But still good theory 😉.

Oxygen isn't what's all in the air that we breathe so I don't think so.

Not true before they found ice and that's just frozen water.