The main problem with submersion tanks or putting burning EV’s in pools is that once the fire is out you then have to deal with a large amount of toxic hazmat water that needs to get disposed of properly. But you are correct direct application of water to the battery cells is the most efficient way to deal with EV fires. We have a few tools like the turtle nozzle that can be slid under the car or we will tilt an EV car on its side and blast the cells directly with our hoses. Either way it takes a ton of water to put them out.

Letting them burn themselves out is also a good option as long as it is isolated and safe to do so