From their own Manuel: FIREFIGHTING USE WATER TO FIGHT A HIGH VOLTAGE BATTERY FIRE. If the battery catches fire, is exposed to high heat, or is generating heat or gases, use large amounts of water to cool the battery. It can take approximately 3,000 gallons (11,356 liters) of water, applied directly to the battery, to fully extinguish and cool down a battery fire; always establish or request an additional water supply. If water is not immediately available, use dry chemicals, CO2, foam, or another typical fire-extinguishing agent to fight the fire until water is available. Apply water directly to the battery. If safety permits, lift or tilt the vehicle for more direct access to the battery. Apply water inside the battery ONLY if a natural opening (such as a vent or opening from a collision) already exists. Do not open the battery for the purpose of cooling it. Extinguish small fires that do not involve the high voltage battery using typical vehicle firefighting procedures. During overhaul, do not make contact with any high voltage components. Always use insulated tools for overhaul. Heat and flames can compromise airbag inflators, stored gas inflation cylinders, gas struts, and other components which can result in an unexpected explosion. Perform an adequate knock down before entering a hot zone. Battery fires can take up to 24 hours to extinguish. Consider allowing the battery to burn while protecting exposures. After all fire and smoke has visibly subsided, a thermal imaging camera can be used to actively measure the temperature of the high voltage battery and monitor the trend of heating or cooling. There must not be fire, smoke, or heating present in the high voltage battery for at least one hour before the vehicle can be released to second responders (such as law enforcement, vehicle transporters, etc.). The battery must be completely cooled before releasing the vehicle to second responders or otherwise leaving the incident. Always advise second responders that there is a risk of battery re-ignition. Second responders may choose to drain excess water out of the vehicle by tilting or repositioning it. This operation can assist in mitigating possible re-ignition. Due to potential re-ignition, a Model S that has been involved in a submersion, fire, or a collision that has compromised the high voltage battery should be stored in an open area at least 50 ft (15 m) from any exposure. Warning: When fire is involved, consider the entire vehicle energized. Always wear full PPE, including a SCBA.

German cities are in the middle of equipping all fire departments with containers that they can flood with water to submerge EVs in. The idea is to let the EV burn, cool it with water, and then tow it under supervision of a fire truck to a fire station to put the EV into water for 1-2 days. Compared to ICE vehicles it's very complicated and binding a fire truck over a span of multiple hours instead of 1 or 2.

Tesla has a relatively comprehensive set of first responder guides on their website. They are all PDFs, which might not help people in the field, but they seem like a good thing to print out and throw in to a binder in every truck. Honestly, if I was tesla I would print, bind, and ship these to every fire dept in the country.

https://www.tesla.com/firstresponders

I have no idea. I'm also not a firefighter. As electric cars become more common, we'll need to have a solution that is not "spend 4 hours trying and then call the manufacturer."

It's not water that puts it out, but low temperature to stop thermal runaway like the FAA advocates for Li-ion battery fires. <0 C salt water would be best.