In the end they say ICE cars catch fire more than EVs. In my European country older diesel cars catch fires and burn quite regularly unfortunately.
You can't smother a lithium battery fire with anything, it will continue to burn even underwater. Your only option is to cool the battery to a point where its below the point of combustion. And that takes considerable time and a lot of water.
This is also happening to relatively new cars not some 30 year old piece of junk.
At least the battery will mostly stay in one place, rather than floating off on the surface of the water, possibly setting things on fire at a considerable distance from the original source.