Well it certainly wouldn't be superheated steam and it's not like it would be a ton of steam in the grand scheme of things. It takes about 2500 kJ of energy to boil a liter of room temperature water. Someone else mentioned 20 kWh of batteries which in a catastrophic failure I'm assuming the cells might release a little more than that amount of energy but for the sake of argument since I don't know exactly what percent to tack onto that we'll just look at the nominal capacity of the cells. 20 kWh is 72 MJ so that's only enough to boil a bit less than 30 liters of water. Submerging a burning lithium ion pack in something like a bathtub full of water is a great way to absorb all of that energy.
Of course, then you run into cooling and maintenance access issues.