From the back of the book:
Conquer the Maelstrom
Sometimes an adventure takes an aquatic turn. You might find yourself exploring an ancient city lost beneath the waves, navigating an underground river plagued by monsters, plotting a course to some stormwracked isle, or battling pirates on the high seas. Well, fear not, brave adventurer. Here is your guide to overcoming such perils!
This supplement examines marine environments, from flooded dungeons and subterranean seas to mystical islands and the gloomy deep. It provides rules for adapting to, navigating through, and surviving hazardous seaborne, underwater, and stormwracked conditions. This book also includes new character options (races, spells, psionic powers, feats, and prestige classes), magic items and monsters associated with seas and storms, adventure sites, and a narrative ship-to-ship combat system.
To use this supplement, a Dungeon Master also needs the Player's Handbook, Dungeon Master's Guide, and Monster Manual. A player needs only the Player's Handbook.
The Kopru Ruins