Any adaptation must find a way to present the world and the story in a new medium. Movies aren't novels, they don't have the luxury of spending pages and pages of prose describing the fine inner workings of the characters and their surroundings in between the narrative or in the copious appendices; it must all be done audio-visually. Through production design, performance and through scoring.
Technically speaking they could indeed just mention or even allude to what the deal is with the navigators...but this is cinema, you're going to want to show *something* otherwise it feels like a cop out, and why would you NOT want to show a freaky weird mutated space-fish-person in your freaky weird sci-fi movie about giant sandworms, witches, and a knife fighting messianic prophet?