As you sink into the water, you still are conscious and will eventually inhale water into your lungs. No act of breathing ever feels as painful as this particular inhale.
According to those who nearly drowned, the process of it feels just as scary as it is agonizing. The entire time you drown, you will be aware of it happening.
And the body's natural reflexes offer no solace on this matter either. As adrenaline takes over, the body remains alert even if your muscle feel fatigued trying to keep you afloat.
The most fateful reflex, to inhale, eventually overpowers whatever willpower you possess. When that happens, it spells the end for you.