Down in the Deep Dark Gulfs

Okay, look, posting this unedited list of nonsense probably wasn't the best use of any of our time, so here's an attempt to tidy it up a bit. Eventually I'll get around to taking each section and expanding my ideas on them a little bit.

Factions
Coral Men living in their Coral cities and Anemone Castles
Translucent Organ Men
Blubber Fish Farms, they aren't good to eat, but its easy to get, the fuckers never try to escape, too fat
Psychic fish what with the translucent head-dome pieces
Trilobite/Horseshoe Crab people
Ghost-Pirates of course, Flying Dutchmen, Davey Jones' Locker, Ghost fleets
Krill-Salesmen (look I don't know, I just like the sounds of it)
Lobster Boys and Crab People, Lobster Wars, and thus, Lobster Knights
White Coral is a Coral Cult

Locations
Sunken Cities from (literally) fallen empires
Whales with Cities/Coral Reefs on their Backs
Whale Graveyards
Ship-wrecks
Flotsam Cities
Geometric Volcano Temples
Coral-Tree Forests
Kelp Jungles
Deep Sea Necromancer Cities
A City Chained to the sea floor, both the top and bottom are used, chain your feet to the floor
Metal + Coral Castle, home of whirlpools and the plug of all the seas
Giant starfish castles
Atlantis
Ur
Green pulses - the lighthouse that guides the drowned dead
Underwater rivers and cliffs
Ys sunk by Dahut's foolishness
Sunken cities that are only 'shallowly' sunk, their buildings poke up from the waters

Monsters
Shark Gods, Kelp Giants
Shoal-Swarms, sometimes they might be people and they throttle you but its just razor sharp fish rushing past you, not throttling you, though they whip the water up so much they may as well be
Smoke-Stack Crabs, you know, the ones on the sea floor, they're on the backs of crabs now
A giant worm that devours the earth and leaves behind these giant trenches for you to explore
EELS SO MANY EELS THEY WRAP AROUND YOU AND THERES LOADS OF THEM HELP
Squid Demons/Flotillas
Whale-Angels
Razored Turtles
Hypnotic Fish Shoals
Axolotl Dragons
Lancer Sharks - like swordfish, but with jousting and lances
Blue Glaucus Frond Fin Dragons (seriously, look these guys up, so pretty)
Space-Bear-Sea-Pig-whatevers
Jelly Fish Collectives
Pearls that turn into Sea Urchin Death Machines like holy shit these fuckers are awful but the pearl makes up their middle so maybe its worth it? Probably not.
Sting-Rays with tentacles and altogether too many eyes
Scylla and Charybdis
Sirens
Typhon
Flameback Sea Slug
Electric Zap Eels
Crown of Thorns Starfish
Gold-Coin predator oysters
Pyrosomes
Telekenetic Brain Coral
Horseshoe Crabs
Geoducks?
Ringed Octopi
Spanish Dancers
Banded Sea Kraits
Cuttlefish are pretty cool
Pufferfish, with actual enlarge/reduce
Leafy Sea Dragons
Pacific Black Dragons
Living Fossils like cealocanths or however you spell it
Scorpion Fish that are actual scorpions
Smalltooth Sawfish
Stoplight Loosejaws (oh god those)
Two-wing flying fish
Whip-tail gulper
Archaic Leviathans
Single Celled Beasts
Corallimorpharia, the eyes of the deeps
Scotoplane sea pigs
Toxic Crabs
Sea Angels - predatory sea snails
Squid Worms urgh
Acatharians Amoeba that are apparently large so now they need to be just absolutely huge and blood-thirsty
Marrus Orthocanna, the physonect siphonophore that I swear I didn't just make up its real life science okay? made up of multiple repeating units, the bio-trains of the sea
Munnopis isopod, armoured snails
Blind furry crabs that filter their food through their furry claws, Kiwidae Crabs - named for the polynesian goddess of shellfish
Sea Spiders with stilt legs and fucking egg sacs oh god help us all
Hydromedusas - jellyfish oh fuck
Tiny Cute Amphipodes
Underwater snakes oh dear
Coral Hydras
Burrowing Sea Mice
Asphichelone, a turtle island that drowns you if you land

Characters
Octopus Oracles in Octopus Gardens
Angler Wizards who carry their lures on the ends of their staffs, covered in barnacles and kelp
Starfish Sorcerers
Crocodile Hashish Smokers
Hags: Benthic, Tentacle, Haze, Primal
Venus but shes alien and terrible to behold
Bioluminescent Gods
Underwater Gods covered in shit and stuff
Iku Turso - finnish god of disease and war
Proteus
Old man of the sea
Qalupalik
The Ancient Mariner

Hazards
Plankton Gases and Poisons
Razor-Clams, seriously, they're weird, and tentacles erupting out of the sand to attack you is cool
Sea Cucumber Tree Trap Things Oh God They're Horrible seriously have you seen them eat?
Clashing Rocks
Milky Sea phenomena
Barriers between Seas
Baltic Sea Anomaly
Brinicles
Red Tide - Deadly
Densovirus that just eats your fucking flesh
What do you get when a whale dies? Vigtorniella Worms
Venus Flytrap anemones, Plankton Clouds
Hungry Clouds
Schools of Fish hiding a horrible killer worm that's got them all strung together as cover for it
Water warps the world it isn't just an optical illusion

Events
Food falls from the sky, open mouths gape in the floor
Crab-Floods and Sting-Ray Clouds
Sea Currents as Weather-with-Consequence, which may also be sentient/underwater ley lines.
Swarms of Sea Stars
Underwater Crop Circles
Underwater sounds, the shouts of drowned gods

Things
Antikytheira Mechanisms
Acheulean, Olduvai, Lithics, bone, caves, pigments, primal weapons
Glow Boats
Squid Penis Flower Egg Sacs
Manganese Spheres
Lattice Tunnels

Other
Sea-as-Dungeon: level as depth, Shallows -> Deeps -> Floor -> Trench -> This is Eden.
Primal Chaos
I AM THE SEA
DO NOT TEST ME
The First
Antedilluvean
Random Octopus generators
Just floating and drifting