Elemental Plane of Water

The Elemental Plane of Water was an Inner Plane or Elemental Plane of the Great Wheel cosmology and the World Tree cosmology models. After the Spellplague, the Elemental Plane of Water collapsed into the Elemental Chaos, mixing with all the other Inner Planes. Water is one of the four elements and two energies that make up the known universe and therefore of great interest to cosmologists. This plane was abundant with life: native creatures born of the elemental nature of the plane itself, sentient water-breathing peoples, and most every species of aquatic life that could survive after being sucked through a vortex from their plane of origin.

Cosmology
According to the Great Wheel cosmology model, the Elemental Plane of Water could be reached via the Ethereal Plane, an adjacent elemental plane, or by an elemental vortex. The Pale River vortex on the Elemental Plane of Earth had its source here, and there was a vortex to the Plane of Air called the Bubble Net. If traveling through the Deep Ethereal, a green curtain of vaporous color indicated the boundary of the Plane of Water's Border Ethereal region. Once in the Border Ethereal, a traveler could observe the Plane of Water and be detected by its denizens. Using the spherical model, this plane was adjacent to the para-elemental planes of Ice and Ooze and the quasi-elemental planes of Steam and Salt. Elemental vortices could occur wherever a high concentration or nearly pure form of an element was found, and could be temporary or permanent. Vortices to the Plane of Water could often be found in the deepest parts of the seas and oceans, in clear underground lakes, or as surface whirlpools in any large body of water. Temporary gates could be created by the plane shift spell or the abilities of high level druids.

As described by the World Tree cosmology model, the Astral Plane connected all planes with the Prime Material Plane and the Ethereal Plane was only used for journeying between locations on the Prime. The Elemental Plane of Water was not connected or coterminous with any other elemental plane, however both the cross-planar rivers Styx and Oceanus were known to have vortices to the Plane of Water. The spell astral projection could be used to reach the Plane of Water via a dark blue color pool. Additionally, the gate and plane shift spells could be used to open a temporary portal to this plane.

Description
"It is an ocean without a surface. It is domain of current and wave. It is a bottomless depth."

- Manual of the Planes

There was no deep or shallow, no dark depths nor wavy surface, just an endless ocean that felt as if you were submerged several feet (say a couple meters) in any body of water on the Prime Material Plane. There was no sun, yet the water itself seemed to glow dimly with a bluish green luminescence. Volumes of water at any temperature and salinity could be found if you knew where to look or had a guide. The Great Wheel cosmology model explained this by the proximity to the para- and quasi-elemental planes: water became cold and formed icebergs as you neared the Plane of Ice; water became brackish as you approached the Plane of Salt; water became silty and slimy as you neared the Plane of Ooze; water started to boil as you approached the Plane of Steam. The World Tree cosmology described this plane as having all varieties of water constantly in motion, influenced by currents and tides. Life that depended on particular conditions flowed along with their preferred environment or suffered the consequences. Impurities such as bubbles of air, chunks of earth, and even short-lived balls of fire could be found floating about due to elemental vortices or the workings of powerful beings. Habitats and settlements typically formed near sources of food and shelter, or near portals and vortices to facilitate trade.

Supporting the teeming life of this plane were the corals and plants that made their way here and found purchase. Huge drifting three-dimensional reefs and loose spheres of freshwater grasses, kelp, and seaweed were home to myriad species and were fertile fishing spots. Travelers had to keep in mind that large predators knew of these fishing grounds also, or else they might discover just how bite-sized they actually were. Just like a Prime ocean, the Elemental Plane of Water seemed to have no limit on how large some creatures could be as giant squid, aboleth, and kraken were known to prowl the plane. Small creatures could be deadly too, with poisonous spines or barbed tails. The smallest of them all was perhaps the deadliest: algae that formed the infamous "red tide". Exposure of the eyes or lungs to the red tide caused a blinding sickness as virulent as any disease.

If the Plane of Water had any weather, it was the currents, whirlpools, tidal bores, and flows of ice, steam, or silt that could inconvenience a traveler or be a deadly surprise. Usually invisible, currents could be strong enough to pull visitors off in some direction for long distances before they were able to exit the current. Tidal bores were the most dangerous currents, hitting like a thrown boulder and carrying the unlucky creature away for miles (kilometers). Whirlpools were caused by countervailing currents that sucked everything in a tightening spiral, some of which lead to vortices to other planes. Ice and silt flows were fairly easy to spot before encountering, but steam flows were nearly undetectable and could cause nasty burns or boil the flesh from your bones.

Inhabitants
It is difficult to determine what type of creatures were the most numerous in the Elemental Plane of Water but presumably the water elementals had the upper hand because they were manifestations of the plane itself. They could take on any shape their fluid bodies could form but they were extremely hard to see, similar to the effect of a robe of blending, and therefore were often described as blurry versions of Prime Material Plane animals and monsters typically of the aquatic variety. Water weirds, an intelligent life form that could possess water elementals, were also thought to be native to this plane.

All other peoples and creatures are interlopers or inadvertent immigrants by way of being sucked through an elemental vortex. Those that made a home here and thrived included some jann, the marids,  the nereids, and the tritons. Besides almost every species of salt- and fresh-water marine life, there were reports of sightings or encounters with many creatures including aboleth, charonadaemons, black and bronze dragons, eyes of the deep, mephits of the ice, ooze, steam, and water varieties, mud-men, sea hags, tojanida, varrdig, and will-o'-wisps.

Trade also brought many different races to the Elemental Plane of Water. Merchants that traded with the dao and the marid included aquatic elves, humans, kuo-toans, lizardfolk, locathah, mercanes, and sahuagin.

Realms

 * Blibdoolpoolp, the Sea Mother of the Kuo-toa, once dwelled in the Elemental Plane of Water.
 * Istishia, The Water Lord, God-King of Water Elementals, once had a realm here.    After the Spellplague he was revealed to be a primordial and he formed a domain in the Elemental Chaos called Cresting Spires.
 * Kalbari al-Durrat al-Amwaj ibn Jari, Padishah of the Marids, Pearl of the Sea, Mother of Foam, Mistress of the Rivers, Savior of Fish, Patron of Waterspouts, and many more titles, had a palace near the Bubble Net called the Citadel of Ten Thousand Pearls.
 * Olhydra, Princess of Evil Water Creatures, once inhabited the ruins of a huge citadel made from black coral. She claimed it was the capital of a great empire that she destroyed long ago.
 * Tefnut, mother of Geb in the Mulhorandi pantheon, was said to live here.