Beware of the locations where can appear more than 3 together. Many players visit Folda to kill one water elemental for fun.
With the Updates/8.61 patch the looting was changed, and you can now open the bodies, BUT it is still possible to "fish" the bodies with a Fishing Rod and get some gems.
All vocations: Remember to bring your Helmet of the Deep when hunting these! It blocks 100% of their strongest attack, the water explosion. The heavy weight of the helmet will be easily compensated as it will save you from their most deadly combos. Knights: Stay away from these unless you feel like wasting to get fast exp. They are very resistant to physical so you should enchant your weapon into Energy to deal normal damage. Skill rings are strongly recommended. Stay away from groups as you will waste a lot of potions for the mighty combos they deal. Even exori gran doesn't hit them for that much damage. Paladins: Take your best bow, Flash Arrows and potions - kill from a distance while being careful to not lure more than two water elementals.
Sorcerers and Druids: Easy creature for mid to high level mages with good magic level. Sorcerer should take Wand of Starstorm, when Druid can take Underworld Rod or no rod at all. It should usually be all to bring them down in no time. Alternatively use HMMs to hit them from afar. They drop strong mana potions quite frequently, which will help you hunting for longer time. Glacier Set will reduce Icicle damage, but it is not neccesary. You will need to heal after each hit to make sure you don't die to a higher combo.