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>!
You have the pure water from the ruins?
Water parts easily, yet put enough of it together and it will create an unstoppable force. It washes away earth and fire, and easily pushes air out of the way.
We are that unstoppable force, <name>, not those that have poisoned Bloodmyst Isle!
Contrary to most quest addons, the water is collected at the Wrathscale Lair, which is on the southeastern side of Bloodmyst Isle.