Jack Vance 
Blue World [EPUB ebook] 

Over twelve generations the descendents of a space crash on a world completely covered in water had managed to adapt to their marine culture. Living in villages built on giant clumps of sea plants, they survived on the flora and fauna of the sea. But they have always been at the mercy of the kragen – gigantic squid-like monsters that prey on their fish flocks, and on them. The biggest of these is King Kragen, with whom the colonists can communicate, who has to be appeased. But one man has had enough of a life of slavery and sacrifice. But how can he convince his fellow men that King Kragen must be killed? And how can that be achieved in a world without weapons?