Gonna write the lore later but important things to know:
Sirens can breathe underwater through their gills.
Sirens can naturally see in the dark (and may or may not have light sensitivity if seadweller).
Sirens have their own sign language called Sirenspeech.
Seadwellers can survive on land but require a LOT of hydration. They can "walk" on lands kinda like a sea lion but without the back flippers.
Siren's gills will close when on land to keep them from drying out