This quest is not available in game.
Fill the Empty Bota Bag and then return to Aqueous in the Hidden Reef at Bloodmyst Isle.
It is worse than I feared. The corruption is very potent and if unchecked it could spread to the ocean currents and be carried throughout all of Azeroth!
We are going to need a sample of the purest water to counter this malignancy. There are ruins on the untainted portion of this island, to the southeast. Go there, where the Wrathscale naga reside, and take from their fountain.
Use this bota bag to gather the sample. Hurry, <name>!