Help me figure out the economy of a Sunless Sea

I've been playing Sunless Sea (videogame) recently and thinking about how an economy would develop in a dark, underground ocean. I've decided some element of the supernatural has to exist, as there isn't really an energy source that would be sustainable (sure, ocean vents could work but I feel they would poison the atmosphere). Assuming the flora and fauna are similar to what you would find on the surface, what could people harvest from the sea in terms of food, basic materials, light sources, etc? What factors should I be thinking about in terms of human survival in a dark, cold, wet environment? Any ideas are appreciated.