Chemists, how is water considered poisonous when over consumed?

This may be as much a biology question as a chemistry question. Water is not toxic in the sense that one would ever call it poisonous, and it's unlikely that you could drink enough of it fast enough to actually harm yourself, but let's say for the sake of argument that someone took a garden hose and shoved it down your throat and started pumping water faster than your kidneys could possibly process it, in addition to upsetting your electrolyte balance (concentration of ions in your body, or salt concentration if you prefer). You'd also be changing the concentration of ever dissolved biochemical in your body. Since reaction rates are dependant on concentration this could create havoc in every system in your body. Hydronium ions, if they exist at all, are not toxic and you'd have a lot of them in your stomach since HCl in the stomach ostensibly reacts with water to form them. Hydroium ions are just protonated water molecules - what you get from acids and water.