I almost died from this as a teenager (not a suicide attempt, just medical abuse when I was inpatient at a residential treatment facility). I was about 115lb at 5'3" at the time, and hadn't eaten anything in a few days, so drinking a large quantity of water basically flushed all the remaining electrolytes out of my system, resulting in significant cardiac problems. It was pretty uncomfortable, I wouldn't say painful, but uncomfortable for sure. I had severe orthostatic intolerance, I couldn't stand or even sit upright without losing consciousness and my hearing and vision kept fading out. I was also very nauseous. If I hadn't been taken to the ER, I would have died that night (that's what the staff there told me, anyways).
I wouldn't really recommend this as a suicide method, mostly because of how uncomfortable it is and the fact that it will take at least several hours for you to die. But it can kill you, and water is easy to access, so there are definitely worse ideas.