Embora eu não consiga encontrar a capa que você mencionou, acho que este é o romance de Jack Vance O Mundo Azul , publicado pela primeira vez em 1966. O resumo do enredo da Wikipedia corresponde aos recursos que você oferece:
Sklar Hast, the protagonist, had achieved a measure of success and prosperity by passing his examination to be a “Hoodwink”, or semaphore tower operator – a prestigious position on the Blue World, a planet with no land at all. During the space of twelve generations, the descendants of a crashed prison ship have created a rudimentary civilization on the water-covered planet, living on huge sea plants. ...
The world is mostly safe. However, they must beware the kragen, giant, semi-intelligent squid-like predators which roam the ocean. The colonists eventually develop a relationship with one of these, King Kragen. It drives off other kragen in return for offerings of food organized by an entrenched quasi-religious priesthood built up over generations. ...
When Sklar questions the need to continue to worship and feed this predator, King Kragen appears, wrecks his home and kills his mentor. Rather than regard this as divine punishment, Sklar suspects that the conservative priesthood has enough control over King Kragen to kill those who oppose their views, and to thus uphold their privileged status.