The sea might seem like it contains strange and monstrous animals. The plane that is the Writhing Depths is the full realisation of the nightmares you might have. It is a bottomless ocean with tall mountains that only act as home to strange and horrifying creatures. If you stay clear of the landmasses, you are visible in the open water and an easy snack for some beast. If you move closer to the corals, numerous things will reach out for you with their tentacles or poisonous spines. There are no safe places. A glow in the dark water might seem like a lantern of hope, but odds are it's an enormous anglerfish. There are settlements here, primarily by merfolk, but none of them is large and all of them are well hidden. When the Kraken is a very real threat, one tends to hide.
Fauna & Flora
Most plants here are able to eat animals that get too close. Most animals have tentacles, stingers, or something else that allow them to reach for something without getting too close. All of them are very able and eager to kill humans who show up. And they can see through the dark waters.