http://www.skybrary.aero/index.php/P...Crew_Checklist

Bottom line, it's not the "pack" that ignites, it's one of the cells. When one of the cells ignites, it heats up the others which, one by one, will ignite and cause more fire.
Water (or any other non-inflammable liquit) prevents this by cooling down the pack. The ignited cell will run its course and stop burning by itself. Fire extinguishers probably won't do much to put the fire out, what matters is to keep the other cells cool and prevent the fire from spreading while the ignited cells run their course. At least that's what I gather from the article