Published in 2006 by Amherst Media. 219 pages. The authors are a husband and wife team of professional undersea journalists, lecturers, and multimedia producers.
There's much more to underwater photography than keeping your camera dry. In general, you have much less control than you do on land. Your subjects are harder to get to. You have a limited amount of time you can spend down there. Your models probably won't sit still for long. Water is rarely perfectly clear in the wild, so you'll probably have backscatter to deal with. And, especially the purposes of this particular book, light works differently underwater.
This book is all about fixing it in post. The version of Photoshop used in the book is CS2, but the principles also apply to more recent versions.
Many of the chapters deal with topics relevant to digital photography generally; things like setting up a digital workflow, backing up images, using adjustment layers, color and exposure corrections, image sharpening, and working with raw files.
But there are some chapters that are specifically relevant to underwater photography. Chapter 10 provides different strategies for removing backscatter (spots of light created by reflections off particles in the water). Other chapters cover topics like shooting adjustments to illuminate the faces of divers, fixing blooming effects (highlight blowouts from the sun shining down through the water). Some of the later chapter deal with more drastic edits, including changing backgrounds and creating composite images. A chapter title "Underwater Plug-In Applications" sounds more promising than it is--the plugins might be useful to underwater photographers, but they're actually more general Photoshop filters rather than filters designed specifically for underwater photography.
The book is filled with useful illustrations of the techniques discussed and they're presented in an easy-to-follow way.
Overall, this book is a good supplemental book on using Photoshop if you already have something like Martin Edge's more comprehensive guide, with some useful tips specific to shooting underwater.