It's possible to put the fire out by cooling the battery below ignition temperature, and depriving the fire of atmospheric oxygen certainly reduces the combusti

It's possible to put the fire out by cooling the battery below ignition temperature, and depriving the fire of atmospheric oxygen certainly reduces the combustion rate, but the result for firefighters is that an enormous amount of water is required. Typical petroleum fires (gasoline, diesel) can be smothered with foam to put them out. No amount of heat will ignite these fuels without oxygen. Smother a self-oxidizing fire does not extinguish the fire. To remove heat, firefighters spray cold water on the fire, which is incompatible with smothering it (the water and steam push foam away), so the fire has atmospheric oxygen as well as oxidant from the fuel. Dropping the car into a tank of water accomplishes both - the water tank is a heat sink, cooling the car, and it also smothers the fire and removes atmospheric oxygen.