What is this item for?

[SinisterShadows]

i'm wondering if I should sell it?  or should I keep it?

[BeK]

It is used as a crafting agent for certain foods and/or potions. I would just stick it in your bank if I were you . 

[Unya]

Waters and quartz are only directly used by the Merry Potters, who in turn craft them into the Bowls and Refiners used by all the other crafting professions. In this case, Sparkling Water is used by potters to make Premium Tempered [Clay] Bowls. Water always sells for less than quartz and usually isn't worth much. Unless you are a Merry Potter or have a friend you want to craft bowls for you, I'd suggest selling it.

[MichelleZhivago]

As Unya said above, if you have a contract with the correct Crafters' Guild, use them for yourself to make things, otherwise, check the market and see if you can sell them for a bit of silver. :)

[Alysha Hawkeye]

That thing is used to craft Premium Tempered Clay Bowl for Banishing Charm and  Ceremonial Meal, which are used to craft Purification Jar for a certain Windwalk quest.

 

Or you can just craft the Bowls and sells them. It has a better price than Premium Refiner due to limited supply, though harder to sell.

 

Without Merry Potter, however, it is useless for anything else, aside from marketplace.