Essence of Water Farming Locations (Classic - WotLK)
The second most expensive and rare Essence in Classic WoW is Essence of Water. You can farm this essence in two different locations off of enemies, both of which are talked about below as well as fish it out of pools in Azshara. You'll need very high fishing to collect Essence of Water in Azshara and the drop rate will be much lower than farming the locations mentioned below.
In addition to the three farming locations, you can also get Essence of Water through Alchemy by using Transmutation. You can turn both Essence of Undeath and Essence of Earth into Essence of Water. It's actually quite profitable (if you're an Alchemist) to turn Undeath into Water as Undeath is generally so cheap on the Auction House. It drops in Stratholme, a very commonly run dungeon at 60.
Eastern Plaguelands - Essence of Water Farming Location
This is the best location for you to farm Essence of Water due to the fact that there will (usually) always be less people here than at the Felwood farming location. In my personal experience and from what I have heard on many other realms as well, the Felwood location is camped 24/7.
In Eastern Plaguelands there are technically two ponds you can visit and farm Essence of Water, the northern pond is much smaller and only has about 10 spawns but the southern one has probably 25-30 spawns. Aside from Essence of Water, people have to hunt these water elementals for the Poisoned Water quest which starts in Azshara and is part of the Molten Core Aqual Quintessence quest line.
Felwood - Essence of Water Farming Location
You'll find about two dozen water elementals in the northern portion of Felwood where it says Irontree Woods on the map, shown in the screen shot above. The water elementals spawn along the water's edge as well as in the water and continue south into the area where the Graveyard is.
As mentioned above, this location is probably the most camped spot in the entire game for Essence farming. In my experience, there are always one or two people here even at 3 - 5am in the morning when usually barely anyone is playing. If you come to this location and there's too many people here, try the Eastern Plaguelands spot instead.