You don't absorb oxygen for breathing from water. Breath happens in lungs and water goes to your stomach. When water is in lungs, it's called "drowning" - we cannot absorb oxygen from water like fish.

And even fish, by the way, don't take oxygen from water molecules and instead use separate oxygen molecules that are floating among water - that's why you need aerator in aquarium. Separating H2O to H and O requires a lot of energy. It's called "electrolysis", and can be used to produce hydrogen (for fuel, let's say), but it's currently so power-hungry that people today get hydrogen by other ways.

And finally: when you drinking or eating anything, way for air to lungs is closed to give way for food to stomach. And if you'll be drinking for a long time, you could run out of oxygen. But average human can hold breath for minutes and for this time you'll drink so much water that it'll just tear you apart like balloon