Been mainlining Aquaman fics lately because I just rewatched that 'Throne of Atlantis' animated movie and needed more ocean floor action. A lot of fics get stuck on the surface drama with Mera or Arthur's royal angst, which is fine, but the good ones really build out the world beneath. I'm talking about Trench horror stories, forgotten Atlantean ruins with curses, or deep-sea expeditions to the Mariana Trench that even Arthur hasn't fully mapped. There's this one on AO3 called 'The Silent Deep' where a research sub gets trapped below the thermocline, and Arthur has to guide them out while fending off a leviathan that's not exactly hostile, just... curious. It nails the crushing pressure and alien beauty of the abyss in a way the comics sometimes gloss over.
Honestly, skip anything tagged primarily with romance if you're here for adventure. Look for 'Worldbuilding', 'Exploration', or 'The Trench' tags. Some authors have nautical or marine biology backgrounds, and you can tell—they'll describe bioluminescent ecosystems or the acoustics of underwater caves with such creepy detail. Those are the fics that actually make you feel like you're six leagues under.