#nautical setting

dense underwater forests made of kelp and haunted by silent predators

bottomless whirlpools, with no explorer who ventured inside ever returning

sprawling reefs made of crystal corals, the reflections of which create life-like illusions when the sun- and moonlight hits it just right

untethered islands floating freely, able to rearrange, part, and fuse at random

a giant chasm in which no light, no matter how bright, illuminates more than an arm's reach

fields of underwater volcanoes, inhabited by deep sea beings requiring warmth to survive

an eternal storm that roams the oceans, and destroys everything in its path

the giant skeleton of a creature which once roamed the oceans, so vast that its body has created a new biome

a giant lighthouse that still sends signals, even though no one has occupied it in centuries

parts of the coast where the water is permanently stained crimson and every night, ghost ships replay their final battle

submerged ruins that are only uncovered when the tides retreat for a couple hours

Are they suffocating out of water? Or just uncomfortably dry? Perhaps they're pinned to the ground by gravity, unaccustomed to their own weight. Perhaps they try to crawl in search of water, scraping up their elbows, belly and underside of their tail as they drag themselves across dry ground.

!!! Also consider: all fish have a thin slime layer protects them from sun damage. Should merfolk have a comparable slime layer on their scales it would make them even more vulnerable to heatstroke and sun damage if it were scraped off in the process. If they make it back into the water it also makes them more vulnerable to parasites.

an enchanting song that literally takes over the mind and makes the sailor ignore all danger to approach

creating the illusion of a safe harbor, maybe a familiar one, the harbor of their destination, or whatever place it is the sailor wishes be at

creating the illusion that the ocean is unsafe, like imagining storm clouds, heavy waves and winds that are not really there, to make sailors desperate to seek safety on land

distracting sailors from where they are going, making them not even realise when they drift too close to shore

affecting the ship rather than the sailor, taking control of it and steering it towards them. rather than a battle for the mind, it is a battle for control over the boat

merfolk catcalling and heckling sailors and pirates as they sail by: the "siren song" was always just interspecies beef

"Look at that idiot. Bet he can't even eat fish with those blunt teeth."

merman always seeing the same sailor at the edge of a boat, staring wistfully out into the water as though they know there is something else out there... in reality, the sailor just gets seasick easily

trading between humans and merfolk: they hire one another to respectively get valuable resources from land or the bottom of the ocean

ever since relationships between humans and merfolk have improved, it is not uncommon to see nomadic merfolk tribes traveling with human ships in a symbiotic relationship and also friendship of sorts

after a pirate ship sinks in a storm, all the pirates end up adopted like stray pets by the local merfolk, who are figuring out how to keep humans alive as they go

merfolk populations who hear the sailors sing at night and have started singing the same songs underwater

"Is it true that you lot drag humans into the depth to drown and eat them?" "Why would we do that? No, like literally, why? Way to inconsistent as a food source and humans don't even taste good." "... And how do you know that?"

adolescents often pierce their dorsal fins

depending on the species, some can never stop swimming because they need to move to be able to breathe

in the underwater community, they often have responsibilities of a doctor, since no one is better at detecting the injured

incredibly curious with a tendency to get in trouble for it

adolescents often pierce their dorsal fins

several rows of sharp teeth, which they lose regularly and often keep for jewellery, gifts, or a simple keepsake

shark half is covered with rough skin rather than scales

usually rather solitary but often live near communities of other merfolk and are on good terms with them