I think the best drop is from the water elementals outside of the Horde city in Twilight Highlands. It's still a really low drop rate though and is a really high traffic area.
If you don't mind me throwing in some advice, your best 2 options are:
1) Bot Elementium Ore - YOu get fairly good amounts of volatile water.
2) Bot fishing pools - Yes! I actually would say this is your best and safest bet. There is a nice little profile to bot the pools in southern Uldum. It's very inconspicuous (I don't know if I spelled that right, but you get what I tried to spell).