The Great Ocean (The Test of Time Supplement)
From D&D Wiki
An endless ocean, filled with an incredible amount of species but no land anywhere except underneath the waves.
An aquatic plane, ancestral home of the Merfolk and many more underwater species, this plane is one of the most crowded and social of the outer planes. But this is not evident when a visitor first comes to the plane and sees only water, all the life this plane contains lives under the surface of the water, from grand underwater cities of the Merfolk to the nomadic tribes of Sahuagin that raid the Merfolk settlements. A beautiful plane with much to see, but only if one has the ability to stay underwater for long.
Meta-game description: Take "Waterworld" mix it with "Starwars episode 1: Phantom Menace" that part where they go through the core of the planet, take away the boats and humans (jedi/anoying jar-jar), multiply the number of underwater species by 1000 and your getting close.