The Altar of Tidal Mastery is located at the bottom of Lake Elrendar in Ghostlands. [71.3, 14.9]
Using [Bundle of Medallions] on the altar will summon Aquantion and must be killed for the quest  Vanquishing Aquantion.
It's unknow why this statue keeps a striking resemblance with a Naga Sea Witch. We can relate the Highborne inheritance of the High Elves with the Naga race, but this doesn't explain how the Quel'dorei had this statue if they didn't notice the existence of the Naga until the Third War.
There are at least other two identical statues - in Ranazjar Isle, Desolace (Serpent Statue) and an unnamed one in the Echo Isles (), next to the Sea Witch Zar'jira.