What battery chemistry is in the Powerwall, Powerpack, Megapack, Megapack 2 etc.?
It doesn't matter. None of them contain lithium in a form that reacts with water, which was your original implication even if you now try to hide it as you realized how ridiculous it was.
I think differences, the change is to less volatile iron phosphate lithium batteries is seen across the board.
It is true that LFP batteries have higher thermal runaway onset temperature (so smaller risk of failure in small events; but it's still susceptible to chain event) and smaller release of energy during the event. So it's safer (than e.g. NCA or NMC) - but not even by a full order of magnitude. Safety (assuming it's not ruined by slacking on other safety measures "because it's safer") is one argument for using LFP. But really, they are used for other reasons; like cost of raw materials, longer cycle life at large DoD%... The compromise on energy density is not a big problem on stationary packs.
As a firefighter arriving on scene, you say it's irrelevant- just pour on the water and cool it off?
No, I say firefighters do not start googling about the battery chemistry and guessing that maybe it has metallic lithium which reacts with water because floobydust speculates so on the internets.
At least hopefully.
What they need is, like with any large industrial setting, a safety plan, a firefighting plan, which takes into account all the details, like: how to ensure no dangerous voltages are present? Where to spray the water (if at all - maybe it's better to let it burn)? What is the risk level of gases emitted? Risk of firefighters being engulfed if the fire suddenly spreads - how fast can it spread? Water being used to cool the neighbor units to prevent cascading effect, and if so, how? Is there some specific "spray here for emergency cooling" hole? Or just cool the outsides of the containers?
Maybe you could work as a consultant to Vistra or the Monterey Fire Dept.
People more involved in lithium ion chemistry than me most definitely work with fire depts. They don't google for speculation on internet forums.