Drowning is a very scary experience. I came very close to drowning once, and I will never forget what it was like. First you fall out of the boat or whatever. I already knew how to swim at the time, but the scary part was the fact that I couldn't get up.
When you get trapped in the water your body's first instinct is to get air, but it's quite difficult when you can't swim. You try to get air but you can't seem to find it so you start accidentally drinking water. I don't know what it's like beyond that, but I would assume you try to stay up but go under, and end up going unconscious and dying.