A further thought about this: people have been advised to keep out of floodwaters because of the risk of chemicals and pollutants in them, so I wonder if any of these other substances could also corrode EV batteries in the way that salt water does.
I don’t believe it’s corrosion that causes the fires.
It is the amount of sodium ions in the salt water that cause a short circuit of the electricity which abnormally causes the battery to “activate” abnormally and generate extremely high amounts of heat. There is even enough sodium in the crust from evaporated salt water to cause the same problem even after the salt water recedes.
There aren’t enough other electrolytes in the contaminated water to do so. I think most of the problem is from petroleum - based oils and fuels like diesel oil and gasoline.