I won't put water...if it's too cold, could turn ice...if it's in summer time, the bottle water may not be good...
In my trunk there's always umbrella, snow shovel, a big jacket (in case there's minor accident and you are waiting for the tow truck, could be an hour or more before it comes).
This one is for myself only...those recycle bag in case I shop in T&T... LOL
personally I always have a snow brush (the one with a plastic blade to help scraping frosting windows and windshield), a map (as a backup in case phone dies...), my insurance, and a pen
the pen came in very handy last time when I was in a car accident