From the publishers website:
At last, a concise supplement that details nautical adventuring for 4th edition. This book has it all, from ship layouts to nautical weapons, underwater combat, weather conditions, and new monsters to terrorize your players with.
From the introduction of the book:
Back in 1997, a book named Of Ships and the Sea was published by TSR for the 2nd Edition Advanced Dungeons and Dragons game. In that book the designers broke down the various ships they were to present into three categories: coasters, inlanders, and seafarers. This book will present ships using the same breakdown for ease of use. Also, Of Ships and the Sea presented only ships popularly used from early antiquity until the late medieval period. This book will also follow that same format.