there are the Kua-Toa, Mermaids ("""merfolk""") , Triton, Locathah and Sauhagin as fish with human bits or characteristics
Then there's aquatic reskins of surface monsters/races that get their own name:
(and are smart enough to hang out in loose society) Merrow, Scrag, Kapoacinths
Then there's magical creatures that you could talk to but having a city of them seems odd
:Selkie, Nixie, Nereid
Then there's other underwater animals but as a race:
Crabmen, Ixitxachitl, Tako
and while every other humanoid race has got an underwater version at some point , only aquatic elves have shown up on the semiregular"
What in gods uglest child is it possible to do with them?
Ocean based fights are notoriously one sided , so players avoid them and having players go underwater with magic involves either ignoring they are underwater or reminding them constantly they are underwater and neither seems that grand.
The only idea I got is take as many as you can bothered doing , and render them extinct and existing as archeological strata .
As archeological strata you would need to condense it down to a detail and a reason for the players to care (unless you have far more patience and patient players).
(also maybe a reason they died)
Note lands and seas rise and fall so this strata doesn't need to be underwater by any means and you can make the ruins make even less sense because they were 3d spaces and that's why this massive room only has one exit on the ground.
Kua-Toa : Build everything out of defeated ritual enemies turned into coral.
Civilization was unable to adapt to advances in warfare due to obsession with getting captives.
Coral tends to release dangerous unpredictable spiritual energy when destroyed
Triton s: Painted murals celebrating their very present and powerful gods.
Gods ate them and ascended to space.
Numerous god stomach and skin parasites still found in hibernating states around ruins .
Made everything out of magic super-glass. Liked imprisoning people in invisible labyrinths .
Turns out they making the glass out of the Kua-Toa coral and storing the toxic psychic waste in vast chambers that eventually burst make everyone turn themselves inside out with whatever was at hand.
Numerous nigh invisible glass weaponry still around. Also those angry ghost shit storage dumps
still around , trying to colonize the land to avoid the mess from the last 3 peoples/.