In Azshara, there are swirls called 'Patch of Elemental Water', where you can fish up elemental water items: Globe of Water, Elemental Water, and Essence of Water. Located mostly in the Bay of Storms there, these are arguably the highest level, and most profitable, fishing spots in the game.
First, in order to fish in the Bay of Storms, you need a ridiculous level of fishing skill. A total of 420 is needed to guarantee landing what you catch, though you can make the attempt to fish with as low as 350 (I believe), however you will rarely land a fish with that skill level. Use lures and equipment to raise your skill if needed. The best fishing items can be gotten at the Stranglethorn Fishing Extravaganza contest, so you may want to try that...with the items there, you can get your skill to 350 (300 skill, +35 rod, +5 fishing line 'enchant' and +5 each on a hat and boots), there is a +2 enchant for gloves, and lures give up to 100 bonus as well.
The pools that contain the elemental items (called a 'Patch of Elemental Water') are spread through the zone in the coastal area known as the Bay of Storms. The area is patrolled by mid-50 level elite giants (as well as a few other mobs), but they're mostly easily avoided if you aren't too low a level. These pools are in set locations around the bay, and it appears that only one (at most) pool is active at a time.
Labels: elemental pool, essence of water, Fishing, guide, professions