Asklemmy

Sounds painful with a high possibility of breaking bones then drowning while conscious. I'd recommend an inert gas that's not CO2 and readily available, like nitrogen or something (CO2 buildup in the blood is what gives the sensation of suffocation). If you're worried about people finding you and a mess, get an enema and stay a bit dehydrated first, and also ensure your body's found within the first 2-3 days if possible (the first thing your corpse does is shit itself, and rot sets in pretty quickly).

This of course presumes you're making the decision to end yourself while of sound mind, not in some panic, feeling trapped or completely hopeless. There's usually a way out that's not as permanent and can lead to future positive interactions that make continued living worth the pain. That said, I'll never judge someone whose pain outweighs their will to live.