Frank, if you don't mind, I'm going to yoink a bunch of the stuff on the Negative plane for my project on the Inner Planes.
Go for it.
High Adventure in... The Plane of Water!
The Elemental Plane of Water is an endless expanse of relatively static water permeated by a soft ambient light. There is only gravity if you want there to be, and the incompressible medium makes gravitational movement slower than walking. But nonetheless, you can move pretty much anything at the rate of about three and a half miles per hour just by "falling" or "rising" with it. Outside of an occasional "pressure zone" the entire plane is pretty much one giant coastal shallows, with a water pressure at any point about that of being under just a meter of water. The Elemental Plane of Water is also the largest place in all of the D&D multiverse in real terms.
Sure, it is "infinite in all spacial dimensions and time" just like all the other Inner Planes, but it is markedly different in that every
point in the Plane of Water is also a
place. None of it is empty or impassable, it's all just made of water. So you can go and
be anywhere, and you won't be "between" things because the place you will be will be an actually stable location in and of itself that you can put stuff down in or give directions to. Every point. And that means that there are more places to be, and by extension more
stuff than in any of the other planes. Indeed, like how on Earth about 70% of your body is water, and about 70% of the world's surface is water, about 70% of the creatures and structures in the Inner Planes are on the Elemental Plane of Water. And like the oceans of every Prime World - the Plane of Water still gets less press than the other planes because it
is full of water. In general, things on the Elemental Plane of Water stay where they are put, with little in the way of mobility. This means that when there is an air bubble, people can pretty much run around in it without fear that the air will bubble up away from them. Because
there is no up. This also means that disposal of bodily waste is "gross." There is nowhere to "bury" anything, so stuff that comes out of you just sits there accusingly. Fortunately, there are a lot of plants and little animals that will come clean that up, but this process is no nicer to watch on the Plane of Water than it is anywhere else. There are areas where, for whatever reason, the ambient water is flowing with some kind of current. Some of these currents are
incredibly fast, but as a rule they are not that "large" and full mixing doesn't happen. The fresh parts of the endless sea stay fresh and the salty parts stay salty. The hot parts stay hot and the frozen parts stay frozen.
The Marids are, individually speaking, the most hard core of the Genies. However, the Great Padisha of the Citadel of Ten Thousand Pearls is basically just the mayor of a town of one thousand occupants. One thousand occupants where one in five of them can grant frickin
wishes, but just a thousand all the same. You could seriously move around the plane your whole life and
never come within the demesnes of a Marid. Each Marid considers themselves to be royalty and to rule all they survey - which is basically true but functionally meaningless because you normally can only see about 60 feet on the Plane of Water because there's microbes and sand and stuff in the water pretty much everywhere. This contrasts sharply with the Sahuagin empires, some of which are ten thousand miles across (note: this is bigger than the entire Earth, and we're talking
volume rather than
surface area, so some of these empires have populations that measure in the
tens of billions), but which due entirely to the sheer vastness of the plane and the
smallness of any visitor's personal experience of the place (60 feet or so around them and movement as fast as they can sink or swim), it is still entirely likely that you've never heard of any of them.
While the visibility on the plane of water is total crap, the
audibility is intense. Water is nearly incompressible and it's nothing but water forever and ever. Sound pretty much follows the rule that any noise is four times as quiet when at twice the distance, with no additional dampening from the atmosphere. Any noise ever propagates with such totality and speed that to the human visitor it is nothing but a constant deafening roar. Indeed, since sound travels so much faster in water than in air, any non-aquatic visitor needs 10 ranks of listen to even have a hope of locating any sound. Even sounds that are loud or close enough to be distinctly made out sound like they are from “everywhere.” This is not a problem that natives have, and indeed a Sahuagin can locate you by the sound of the water against your skin.
Secession is constant in the Plane of Water. Anyone can just pick up their
house and leave at a bit over 3 miles an hour. Between this tax day and the next, you could have moved your house about 29,000 miles – which is noticeably more than the circumference of the Earth. And when you factor in the fact that there is no guaranty that anyone will find your house if you move it 100
meters, one can see that you can vanish from a government's radar
very easily if you are not actively imprisoned. The standard therefore is to be required to pay taxes to the local authorities at the
beginning of the year and subsequently be allowed to provide proof of citizenship to receive services for the following year. Surprisingly, much of the civilization in the Plane of Water is actually
more recognizable by connoisseurs of modern nationalism than are the kingdoms of other planes of existence. If you want to live in a “country”, you have a citizenship card and rights and social services and stuff. Anyone who doesn't want those things (or doesn't want to pay for them), just
leaves and lives elsewhere in the roaring darkness.
Campaign Seed: Heralds of the Empire: Sound travels fast under water, but news does not. When a new nation takes hold of a region, it can take a long time to even
find everyone who lives there. And so it is that any nation state or empire needs to send out groups to patrol their territory. Not just to keep an eye on the citizens and provide whatever services the empire provides to the hinterlands – but also to keep the maps updated. After all, any part of the empire that hasn't been patrolled in the last
month could seriously have had someone move a castle from 4000 kilometers away to there in the meantime. As representatives of the state being sent into areas of water that the state either has not been to
yet or has not been to
recently, the PCs could encounter pretty much anything at all. And they have a built-in plot hook that encourages them to interact with anything they fine. Whether they face level appropriate wandering monsters, social encounters with dubious locathah, or hostile empires coming the other way, the PCs can plausibly encounter level appropriate opposition at any level.
Campaign Seed: Tidal Merchants: The great tidal streams are currents that move with surpassing speed. Those who ride them can get places that are
very far away in very short periods of time. And that's saying something in a world where seriously anyone can tie themselves to their cargo and “sink” 80 miles a day just by deciding to. The currents don't just provide fast transport, they also provide a path, a place to go. And so it is no surprise that as one drifts along the tidal stream, one can hear the drums of civilization from all sides just as you can see the glowing lights of fast food joints while driving on a freeway on Earth. Traveling along the tidal streams brings one from one urban development to another with all the vast spaces between literally washed away.
Ten Low Level Adventures in the Plane of Water:
The old Locathah is certainly
interested in your proposal. But he says he has other problems...
- Sahuagin raiding has hit several nearby kelp farms.
- Shark attacks are on the rise.
- No one seems to want to buy the sponges he has been growing.
- His daughter has the ick.
- Food supplies are running low.
- The fish are migrating out.
- A local hot spot is attributed to Fire leakage.
- Those who die seem to come back as zombies.
- A siren has been throwing her weight around.
- Pirates have seized the oyster bed.
Ten Mid Level Adventures in the Plane of Water:
The sound of drums has called you to the activities like moths to a flame. When it comes into view, it appears to be:
- A brass sphere, with no immediately obvious entrances.
- An army of skeletons.
- The coral towers of a merfolk city, they look sick.
- An ice factory.
- Angry tritons.
- A giant eel that had been mimicking civilization sounds by slapping rocks together.
- A Sahuagin kelp outpost.
- A family of scrag wreckers.
- A Marid Sattrapi
- Some sort of mechanical vessel shaped like a lobster.
Ten High Level Adventures in the Plane of Water:
You've broken into the massive mechanical manta ship. Inside you find...
- Spongy, organic passageways... this ship is alive.
- The crew are long dead and dust.
- The captain's log mentions you by name.
- Kuo-Toan pirates and their Yugoloth servants.
- Sack after sack of dream dust.
- These look like dragon eggs.
- The spectral pirates who run this thing.
- A cargo hold full of wild eyed prisoners.
- A cargo hold full of non aquatic and fearful prisoners.
- The ship's wizard captain and his crew of blood-indifferent golem pirates.