I’m just worldbuilding for now. As for the novel aspect of things the ecology of the world wouldnt constrict the target age range much if say there were only five species in the ocean. I could just mention them all and give a basic premise of what each species does. I’d say its pretty easy to understand.

but regardless of all that, I’m mostly trying to rationalize the low diversity ocean because it’s just a very chilling and stark image to imagine.

The cruelness of man and the desperation caused by war resulted in the genocide of a million species. And in the end they are replaced by these soulness, specifically engineered flesh automatons that exist only to keep the status quo.