d20 Aquatic powers and dwellers of the deep

We turn once again to the magic of Azgaars and something that occured to me as I was fiddling with the map - and an offhand comment on an old OSR blogroll made me think - what about the oceans? In D&D, the seas are stuffed with sentients, nations, powers of all sorts - for a coastal realm they need to be accounted for and certainly people ought to have some inkling of what is down there in the dockside taverns and so on.

Looking at just our relief map of the world we have a bunch of rivers draining into a shallow sea immediately off our campaign and then a deep but narrow ocean beyond that. There is certainly space for some things to be lurking.

We have four distinct spaces just in this map - the shallow 'bay' fed by all the rivers, the somewhat deeper narrow sea to the south, a true sea in the centre with a good deep centre and beyond to the east the deep ocean.

Taking a look at wikipedia we should have three zones stacked vertically:
- the Sunlight zone - all the food - plants, grazers, predators
- the Twilight zone- low light, cold - where herbivores lurk for safety
- the Midnight zone - no light, crushing pressure, cold

For the most part life and interesting things will only be found where there is food so the Sunlight zone or within reach of it or in pockets along the deeper ocean floor around some anomaly like a whale-fall or thermal vent.

What is interesting for our purpose is to think on who, if anyone, might be down there.

d20 Aquatic powers and dwellers of the deep:
1. Classic Aquatics - merfolk, tritons, merrow - are going to be sunlight zoners mostly, since what they eat also comes from the sunlight zone
2. Fishfolk: kuo-toa, locathah - Border crossers able to live in sunlight and mine in the twilight or deeper zones. Similarly, tough aquatics like Sahuagin might lair in the twilight or deeper and hunt in the sunlight zone
3. Undead - without need of food, these could be found even in the deeps - both unintelligent skeletons and zombies and potentially quiet-seeking liches
4. Fiends - the aquatic fiends that roam the river Styx will all be available for deep aquatic wickedness - and since there are lots of souls down here in all the aquatic races, one imagines there will be aquatic variants of erinyes, succubi and other fiends to tempt and corrupt them
5. Fey - from undine to hags off through various other water-nymphs, the fey should be present in the oceans - perhaps more concentrated in the bright, threshing spray of the upper zones but in the dark deeps one should find connections to the shadowfell and hags
6. Illithids - all the aquatic races can likely be ceremorphised so I would expect illithids hatched from merfolk, tritons and other near-human physique creatures. Found in the deepest and most remote parts of the ocean floor, with captive populations trapped in caves or in submarine caves that connect to the deep underdark
7. Dragons - dragon turtles in the deeps, other dragons may lair on the surface but roam the upper reaches either by swimming or by flying and diving - bronze and blue being typical
8. Aquatic insectoids / crustaceans - chuul are the classic - presented as cultureless wandering monsters, one could imagine that since we see swarm-like behaviour from crabs of all sorts, there are chuul-like entities out there that cooperate and build communities.
9. Aquatic underdarkers - Ixitxachitl - manta like dwellers of coral reefs, usually followers of Demogorgon, despoilers of all around them
10. Marids - the water genie, similar to fey, would seem to most likely prefer the shallows given their aesthetic sensibility and the muting that happens to all colours in the deeps. If they have holdings in the deeps it would be for wealth-harvesting or other sensation related thing - brine pools or hot-vents to lounge at or Peaceful sanctums for contemplation rather than residencies
11. Aboleths - were here before everything else, are still down in the deeps, dreaming away and hatching their strange schemes. One can well imagine aboleth holdings were once somewhere else entirely but the drift of continental plates has brought them deeper/shallower and so they could be anywhere for no immediately apparent reason
12. Beholders - eye of the deeps - driven by beholders classic urge to dominate and conquer, spice with a side of spiting any other beholder that might claim something left untaken. I could see beholders both holding strange deeps for their own strange investigations and also simply to deny it to others.
13. Celestials - deep streams of the oceanus, connections to the oceans of the upper planes, sacred places to various sea gods - all these could lead to celestials being present as guardians and guides in the deep
14. Sea-Tortles, Lizardfolk and other aquatic reptiles - the standard tortle walks but that their mer-analogue should exist makes sense. Another bunch much like mer-folk and tritons, nomadic dwellers of the sunlight zone, but their need to breathe air keeps them tied to the surface even as they are comfortable diving deeply.
15. Ghouls and sea-trolls - though one undead and the other not, both these scourges are a blight upon the sea-floors and the short distances they can clumsily swim. Easily out-swum if seen, they lurk in ambush around things that would tempt others closer
16. Gestalt-corals - similar to myconids, but aquatic and fixed in place, they form great communities that defend themselves with whipping stings and fearsome pets. Often bent to the use of more powerful beings despite these
17. Leviathans / Megalodons - whales, sharks and other enormous beings that may have territory but typically no lair as they roam the open waters and cross great distances at whim - often the source of glorious quests or fearful monitoring like a natural disaster
18. Kraken - exemplars of other great monsters who take lairs - they will have staked out a zone around their lair as opposed to the other great beasts that migrate - whether they demand tribute or just attack things randomly will vary - may well have vassals / cultists and be treated as a minor sea god.
19. Tako - these octopus folk are among the best able to deal with depth, often with fortified dwellings with tiny entrances difficult for non-cephalopods to squeeze through. Their malleable forms and camoflage abilities makes them well adapted to monitoring the activities of others and they are often the heralds to other groups of trouble entering a region. Bitter foes of the illithids as their distributed brains make them more vulnerable to mind blasts,
20. Giants - both sea giants and storm giants are happy in the deeps and may bring a surprising surface aesthetic with them, combing wrecks for treasures that other deep dwellers would disregard - furniture for instance.