Throne of the Tides is part of the player-accessible section of the Abyssal Maw complex. It is a 5-player underwater dungeon, with distinctive architecture--including a liquid elevator. It is one of the first Cataclysm dungeons players can queue up for in Normal Mode. In Throne, players battle their way not only through the host of naga that controls the place, but through servants of the Old Gods themselves as well. There were plans to develop the lore in this dungeon in 4.1, but that was scrapped in favor of further developing Firelands.

Players who complete Waters of Elune with a Highborne Scroll grant their party Waters of Elune inside the dungeon.

From the Blizzard Community site:
"Deep within the Abyssal Maw's fathomless expanses lies the Throne of the Tides. From here, the great elemental lord Neptulon the Tidehunter has surveyed and protected his aquatic domain for ages. He now faces his greatest challenge as a force of serpentine naga and malefic faceless ones threaten to depose him, leaving his realm and its secrets ripe for the plundering.

It seems preposterous to think that even the armies of the naga would challenge an demigod on his home plane - and, yet, they are winning. Instead of cowering before Neptulon, the naga have brought forth a gigantic ally to hobble the Tidehunter...one who holds the entire Throne of the Tides in its grasp.

If their siege isn't broken, Neptulon will fall - and the naga will rise in his place as the new lords of Azeroth's oceans."