Let’s not forget mushroom coral shields, brain coral helms, fire coral tridents, giant urchin armor, and the like.
An undersea fortress has many advantages and disadvantages, which build upon its alien nature. Certain sea sponges thrive on waste, so it seems likely that undersea races might use them for disposing of organic wastes. Certain fish and dinoflagellates produce bioluminescence, which aquan races might utilize. Every aspect of a typical dungeon crawl can be reevaluated, when it comes to undersea adventures.
In my own campaign, an ancient civilization raised their island home high above the sea, suspended by giant columns. They lived in synergy with the sea, enhancing and contributing to the local flora and fauna. They developed translucent stingrays, known as plasmantas, which emit blue actinic light, to aid the growth of nearby corals. They created the gorgonians, a race of living sponges, to filter pollutants from the surrounding seas. The engineered the skumdusters, an odd worm that feeds on poisonous nitrates.
The arrogance of sorcerers, the whims of nature, and the wrath of demons brought the island down. But the creations of the Skyland wizards remain.
Admittedly, having 750 gallons in saltwater aquariums gives me a head start, when it comes to envisioning an aquatic campaign. I can sit beside my 250 gallon reef tank and watch a lone sea star or clownfish wind its way through passages of coralline algae-encrusted liverock. I am often fascinated by the antics of copepods; tiny shrimp smaller than a grain of rice. Or, I can look below, at protein skimmers, calcium reactors, wet/dry filters, and the like, and devise living equivalents for a fantasy realm.
Or, for a change of pace, I can spend some time with my 9 birds, as inspiration for the races of aarakocra, harpies, and dragons which live in the world of Those Above.