one deep breath and lights out as you pass out. probably don’t hurt at all, just pass out.
One of my friends “drowned” when he was twelve years old, or so. They pulled him out of the water and were able to revive him. He told me the drowning process itself was horribly excruciatingly painful. Especially at first. Then there was the fear that made it worse. But just before he lost consciousness, he suddenly gave up and relaxed; he wasn’t afraid anymore and he wasn’t in pain, just at peace with it, almost euphoric.
I remember a scene from a book I read years ago, in which a guy is dropped into the East River with cement shoes, but the killers put a scuba tank on the victim and taped the mouthpiece in, so the victim would have an hour or so to think things over.