Skettis, Terrokar Forest
There are two locations for Primal Water farming in Terrokar Forest. The primary, and probably the best spot overall, is Skettis. There are lots of high-level Water Elementals in the Skettis Lake. Other than that, the Eels in the River near Shattrath also drop Mote of Water, though their drop rate is lower.
Primal Water is all over Nagrand
There are tons of Water Elementals in Nagrand, which makes it a very good spot for farming Primal Water. If one of the spots is too full of people, you can just switch locations. The drop rate on the Elemental Plateau in the north-east is the highest of the zone, though there will always be people there.
Coilskar Point, Shadowmoon Valley
While there are not a lot of Elementals here, Primal Water can still be acquired in Shadowmoon Valley. The elementals are around the watery areas of the location. They do have a slightly higher drop rate than the lower-level alternatives.
The Lagoon, Zangarmarsh
The Eels in the Lagoon of the Zangarmarsh drop Primal Water as well, which not many people know. In Addition, the Bloodthirsty Marshfangs around the Lagoon can also drop Motes of Water. This makes the spot great for farming Primal Water at lower levels.