I have come very close to drowning , so when I went to the amusement park when I was a kid, I jumped i to the pool, and I couldnt find my footing and I was under water panicking, my eyes were closed and I could here nothing but the water splashing and maybe even that I could not hear properly, all I was focusing on was to somehow stand up, you dont think of anything in situation like this, then I kept my cool then I managed to bring myself back up. Then I saw a lifeguard just stainding right there, doing nothing.
So during drowning there is no thought of breathing but the survival mechanism will make you stop breathing, i felt a little blood pump in the head and chest, but during drowing one cannot breathe in, the body shuts down the breathing mechanism.
also you wont be able to exhale, the body literally does the hand waving and stuff instinctually.
Lets say i stayed underwater for another couple of seconds , I would blackpout, faint, and then the body will breathe in involuntarily and then the water fills the lungs and stuff and death will occur due to lack of oxygen.
This method can be mixed with other like alcohol consumption, with sleeping pills or sedative.
Ideally the best way will be to use anasthesia .or blood released untill blackout or using inert gas.