From the back of the book:
The ocean is perilous.
Powerful winds die can die, leaving sails limp and ships drifting listlessly on the vast blue brine. Yet in less than an hour a sailor can be torn from the doldrums by a violent tempest ripping the ship apart. Should a lifeboat survive the storm, the great waves in its wake will spray mists of chill and fear on huddled survivors. Current and wind will be the only hope left for a safe destination. As the small craft is jostled north, south, east and west; the little boat will end up in the one direction that is certain — down.
Down where terrible creatures underneath the waves await. There the mythological monsters and inhuman horrors thrive amidst liquid poison and steel-crushing pressure but delight in assaulting the ships, boats and rafts which skirt surface of their domains. The most feared of these are the sea-devils. The Sahuagin.
Sahuagin are a predatory, piscine race that emerge from the ocean's deepest trenches to prey on ships and seaside communities. The sahuagins believe the entire aquatic realm is theirs to rule. They are taught that it is only a matter of time until their shark god delivers every living creature, both on land and at sea, to their toothy maws.