That could indeed be a solution. Enough mass of water or enough flow of water will transfer the heat away fast enough. The battery is done at that point anyway

That could indeed be a solution. Enough mass of water or enough flow of water will transfer the heat away fast enough. The battery is done at that point anyway, but submerging it will stop flames and toxic fumes from forming from an "open" fire. The cells will "melt" and react with each other on and on until they're done, you can't really stop that. The same thing is done with gas bottles. Once they have been subject to flames or enough heat, they are considered unsafe and will have reactions going on in the inside. You can measure those due to a rise in temperature on the outside, too. Gas bottles are also put into containers full of water, for at least 24 hours, until the reactions on the inside are done. Cooling them prevents them from heating up until they burst though. Same goes for EV battery packs.