On the first day, the gods separated the sea from the land.
That was just the first mistake.
The world you know is a massive, single ocean littered with hundreds of small islands. The primitive tribes that live there have scraped a living off of the rocky shores for longer than history records. They are simple, honorable folk, but a superstitious lot and quick to violence when it comes to protecting what little they have. This was your life.
Then the sea people came.
The raiders razed town after town and with them came vicious creatures from the depths - creatures of claw and shell and scale who sang hymns to their mad sea god as they dragged themselves from the foam. The tribes-people cried out to their ancestors, pleading with the gods to teach them how to resist the marauding sea raiders. To some the answer was as painful as the problem.
The tribes must unite.