My opinion is that water is a good choice for a battery fire, EXCEPT where the pack is high-voltage, and water could lead to electrocution of people involved.
The Leaf, the Rav4, and the Tesla battery packs are all contained in hermetic metal cases. (the volt apparently uses plastic) You cannot get water into these cases easily, and I would hesitate to puncture the case (thus admitting oxygen).
Unless one of these cars is involved in a severe collision, I don't see any concern over a fire. If you are in a severe enough collision to warrant concern for the battery, you likely won't be alive to care. In the unlikely event you ever witness your pack smoking/swelling/venting, get the car safely outside if you can, and then stand back. I would not bother trying to get water on it.
In the Leaf, even if the traction pack is removed/disconnected, the 12v battery will still allow you to shift the car into N as long as it has charge and the electrical system is intact.
Bottom line is you are probably a million times more likely to die in a car accident than to suffer a catastrophic battery fire.
-Phil