I am really not sure why the FAA say that. I frankly think they have not studied it enough. I understand not all lithium polymers are the same and I understand that I am no expert on it. None the less in hobby circles (yea I am a techno geek) water on lithium's and some fire extinguishers are taboo and only create more instant combustion.
I understand the desire to cool the batteries to stop the runaway. I also think without more accidents they will leave the recommendations there.
Do a google for lithium battery fire water. There are guys videos of people cutting open batteries (not in thermal runaway) and dropping parts in water to instant ignition; which brings even more questions to how TSA...........
Personally I would like to see a commission to study the dangers and the alternative containment.