There's plenty of adventure hidden under the gently lapping waves and crashing seas of Golarion. Many land-dwellers don't realize that a rich society and ecology exists in the briny depths of Golarion's oceans and seas. In this book you can learn more about merfolk nations and the dangerous sahuagin and other aquatic terrors that wage war with them and other peaceful aquatic humanoids. Find lost treasures and explore strange underwater cities within these pages. Furthermore, this book provides a wealth of rules for underwater combat and ways for terrestrial adventurers to adapt to an underwater environment, including new archetypes, feats, and magic items.