From the back cover:
The Seafarer's Handbook is the essential d20 System resource for aquatic campaigns and high fantasy adventure. This sourcebook is packed full of new feats, skills, classes, races, and magic for characters to utilize whether on land or under the waves. DMs are provided new rules for ship-to-ship combat, navigation on the high seas, and a variety of colorful locales in which to place a sea-based campaign.
Three new PC races: aquatic elves, half-merrow, and merfolk
Detailed rules for using existing skills and feats underwater
More than 30 new skills and feats to suit an aquatic campaign
New weapons, armor, and equipment for use in, on, and under the waves
More than 50 new spells and magic items
Extensive new rules for underwater combat
A chapter on running seafaring campaigns
A detailed chapter on undersea adventures, including new monsters to challenge seafaring characters
Two full chapters on ship design and construction, including 20 fully detailed and illustrated ships ranging from the dragon turtle-class troop transport to the dwarven ironback
Extensive ship-to-ship combat system, including rules for crew combats and swashbuckling adventure