I drowned when I was three. Can't remember much, mostly because of my age and/or possible trauma. Last thing I remember was kneeling at the side of the pool trying to fill one of my toys with water. That's all. My friend then ran around the house to the front garden to fetch my mum. By the time she got there I was already floating face down in the water unconscious. Can't have been more than a minute or two. The doctors later said that I must've been screaming in panic (couldn't swim at the time) and therefore took in the water immediately and passed out.
Since I can't recall, I can't tell you what it feels like, but I did consider this method for a while before coming here, it just seemed the logical exit. Thought about different ways to dive below the surface, see the sun above (always thought that was beautiful) and end it there somehow. Even considered a block of concrete chained to my leg. Plus, it seemed an appealing way of getting rid of my body.
From what I've learned, just as was in my case, if one gets the water in one's lungs it's rather quick and not too painful, since one naturally passes out at that stage and the rest happens while one is unconscious. But that's the big caveat (!), and one can't stress this enough, it's not that simple doing this and it'll probably be agony, similar to holding one's breath, before getting to that point. Would heavily intoxicate myself for sure to lessen the sheer terror and then try to take a big gasp or alternatively devise a means of deep unconsciousness beforehand and then drown.
The following is a drowning scene from the Japanese film Harmonium. It's beautiful and how I always imagined it. It's not graphic, but be sure to use headphones or proper speakers, it's a waste otherwise. Password is: dive