AKA: The Bottomless Deep
The Plane of Water is an infinite expanse of liquid, without surface or bottom, a forever sea mixing fresh water, salt water, light areas, dark areas, and all other aquatic environments into one vast ocean, without beginning or end.
The Plane of Water is fairly temperate, sweet, and sunny on the whole. Certain regions are cold or warm, salty or fresh, dark or light, "surface" or "deep," but by and large the plane is very hospitable to any entity that can swim. Breathing is something more of a problem, but once a traveler can take a breath, the plane itself presents very few difficulties, especially if you find a swimming mount such as a hippocampus or a sea horse.
Currents can sweep travelers to and fro, and especially strong or persistant currents might disrupt the plans of anyone who isn't a powerful swimmer. Related, whirlpools formed of swirling currents can trap a traveler, whirl them around, and bash them with debris, for hours, days, or even years.
"Burn Water" is acidic water. It is transparent, and otherwise difficult to distinguish from the background water, until one blunders into it and it burns their face off (along with a big chunk of their equipment).
"Red Tides" are tainted currents that carry microbial disease. Those that breathe water are more harshly affected, frequently dying from the cocktail of infections and illnesses that can result from it. However, even touching this crimson seawater can be damaging, and becoming infected carries a long, painful convalescence.
Most normal aquatic creatures can be found in various regions of the Plane of Water. However, air-breathing aquatic creatures like whales, dolphins, seals, and the like, are rarer, given the general lack of air on the plane. Still, enough open regions exist that these creatures are not entirely absent.
More dangerous aquatic creatures -- sea monsters, denizens of the depths, and wicked, immense entities of the abyssal plains -- can be found in various corners of the realm as well. The most well-known are the water weirds, who dwell in the thickest concentration near the saline border of the plane with the Plane of Salt, and who also serve Olhydra in great numbers.
Aquatic peoples of various stripes form a bickering patchwork of kingdoms, fiefdoms, empires, and dominions. The various villages of trolls, giants, sahagin, tritons, merfolk, etc., folk keep largely to themselves, only interacting with others to trade or to war.
Water Elementals are the most numerous residents of the Plane of Water, living here as commoners, building kingdoms and empires, and living lives in service to each other, and to the elemental lords of water.
Marids, a kind of genie, dwell here as well, and form one of the greatest cities on the plane, the Citadel of Ten Thousand Pearls. They are arrogant and proud, but magnamonious. They fight evil wherever they find it, and while they generally consider themselves as not needing alliances, those who favorably impress them may find themselves helped with the collossal magical powers of these creatures.
Ahto, the King of Seas, is the Finnish god of the Sea, forms a strong presence on the plane. He protects the creatures of the sea, and is served by giants, a particularly clever bard, and a dwarf made entirely of rippling water.
Ben-Hadar is the boisterous, arrogant, rude, and uncouth Prince of Good Water Creatures. He is largely considered insufferable, but his protection is valued, as he is a powerful warrior who truly cares about his charges (though he doesn't much care what they think of him). He rules from his fortress of coral, Ssesurgass.
Blibdoolpoolp is the goddess of the mad, desperate, vengeful kuo-toa. Few of her people are found in the lit waters of this plane (more are perhaps found in Pandemonium), but she broods and plots and rants and raves here anyway, in a dark little abyssal place known as the Murky Depths.
Eadro is the shared deity of many fishy creatures, especially locathah and merfolk. His domain of Shelluria is a bountiful and beautiful place, filled with gossamer schools of transparent fish that ripple with all the colors of the rainbow. Rumor has it, however, that he hasn't been home in decades, and none are quite sure where he went or what he planned on accomplishing before he returned.
Istishia is an "honorary elemenal prince," a creature said to make up the entirety of the Plane of Water himself. Viewed as synonymous with water itself, every drop is said to be a tear or a drop of blood from this creature. Many water elementals- pay him homage, though he doesn't seem to manifest often, and when he does, he is small. He doesn't appear to have any major goals aside from keeping Olhydra and Ben-Hadar from declaring dominance of the plane.
Olhydra herself is the Princess of Evil Water Creatures, and she is as close to a deity as an elemental royal gets, being worshiped by scores of people of many different faiths all over the planes. She broods in her coral castle, seeking primarily the destruction and extinguishing of Imix, the Prince of Evil Fire Creatures.
The Blue Coven is a trio of sea hags that has become a servant of Olhydra's. Sharing nearly one mind and one will between the three of them, they act as one mind with three bodies rather than three different creatures. They act in service of Olhydra and her goals, though they have provoked the ire of the champion of hags, Cegilune.
Coral Rivers are present throughout the plane as snaky, winding paths of free-floating coral. These are often centers of civilzation and culture, used to anchor floating palaces and to harvest raw materials from, in the great emptiness of the Bottomless Deep.
The Avenger is a construct resembling a manta ray, that shocks anyone who gets near with electric bolts. It may be linked to the insane artificer, Lum the Mad.
The Citadel of Ten Thousand Pearls is the magnificent city of the marid, ruled by the Great Padisha of the Marids, Kalbari al-Durrat al-Amwaj ibn Jari. It is rich and flowing, with strings of spherical lights, glistening fish, waving kelp, and brilliant shell-like architecture forming the beautiful surroundings of the Padisha, who delights in spectacle. Though she seems quite childlike and excitable, many keen eyes have noted her political scheming rivals the devils.
The City of Glass is a planar metropolis inhabited by all manner of aquatic life. It features an air bubble that enables travelers to come here and spend time (and money). It serves as the major trade hub of the elemental planes, and creatures from all over the planes come here to trade in elemental wares.
- In Muspelhiem, on Ysgard, a wide, shallow pool of water contains a portal. This portal is guarded by giants, but opens for a long period of time, about an hour, allowing a significant amount of crossing between the planes while it remains open. It is known as the Silver Eye, and the giants use it as a divining pool.
- The River Oceanus, in Elysium, has a gate guarded by an elemental specialist, and by many guardinals.
- In the mortal world, storm surges and whirlpools can contain spontaneous links to the Plane of Water. Whirlpools on the plane sometimes have portals that return someone trapped in them back again to the mortal world.