The only way to ensure the water kills you, is to maximize the surface area of impact.
If you're going to suicide in a pool of water...make sure you perfect your divining technique. Otherwise you'll wake up in the hostipal with a prolapsed asshole, never able to shit normally, pissed off that you're still here on earth.
If you're going to jump, land on concrete, because water IS NOT 100% kill proof no matter the height. Water impact will only kill you if you break the surface tension in the least efficient way. 9/10 jumping into water leaves the victim reying on others to empty their shit bag, but still alive.