Tesla's guidance for dealing with battery fires in their cars is to basically flood them. I think the idea is to keep the temperature down. The water won't extinguish it because it's a chemical fire, but you can use water to keep it from spiraling out of control.

Yes, it’s to keep the temperature down to not start secondary fires. You cannot stop thermal runway and must let it take its course.

However dumping water on mass amounts of stored electricity sounds scary.