The zone is Tol Borad. While you do have to be level 85 to get here, there has been some talk about being able to fly here if you aren't, it's especially easy with the level 3 guild perk. Any who, below is a map that shows you the route that should be taken, keep in mind that the herbs are underwater. When I farm this spot, I generally carry 30-40 water walking elixirs on me, this allows me to walk on water and dive down to grab the herb. I've noticed that the crabs in the water are very easy to dodge and maneuver around.
I can gather 3-4 stacks in under 10 minutes using this method. The best thing is that this herb is so rare it still is selling for a nice price, I currently sell them for 395g per stack, and they sell and quickly.