Speak with Dendrite Starblaze in the village of Nighthaven, Moonglade.
[player], I have been following your progress with much delight. You are most assuredly developing yourself into a valued protector of nature.
The time has come for you to learn more about one of the various animal aspects that Cenarius has blessed us with the ability to change into - one that affords you command and mastery of the water. For this, you must travel to the village of Nighthaven in Moonglade and speak with Dendrite Starblaze of the Cenarion Circle there. He will guide you further.