I always find the merfolk genre does two things really well: it makes the ocean feel vast and ancient, and also deeply personal. The best ones, like some of the darker fantasy romance plots out there, use the underwater setting to explore societal structures that feel alien but oddly familiar. You get these intricate caste systems based on coral districts or depth zones, political intrigue over territory, and a sense of history that's literally layered in silt and shipwrecks.
A story that stuck with me had a mer protagonist who was essentially a royal archivist, and her world was built through the slow accumulation of knowledge in sunken libraries. The sea wasn't just a blue backdrop; it was a character with moods—the crushing silence of the abyss, the frantic energy of a predator's chase, the eerie glow of bioluminescent forests. It showed how life down there operates on entirely different senses and laws, which then directly fuels the conflict when humans blunder in with their noise and light.
The physicality of being a merperson often gets glossed over, but when it's done right, it adds so much. The struggle against currents, the way communication might involve sonar or color shifts in scales, the constant negotiation with a medium that's both supportive and perilous. It turns the environment from a setting into a narrative engine.