The Blood of the Ruler

You are a marine biologist that heard of a myth about the waters you're studying, so you decide to descend into the waters to find out if it's true. And guess what, it is true, there's a whole society down below the waves in a cave. But in a twist of fate, a murder has happened, someone killed the ruler of this place, so, to leave, you now need to solve who killed them to be permitted to leave. Good luck!