At the risk of sounding repetitive, I would heartily recommend Martin's book as well (The Underwater Photographer, Third Edition: Digital and Traditional Techniques by Martin Edge). I bought it thinking that he would be mostly talking about SLR's (digital and film), but he also covers Point and Shoot cameras and the associated techniques as well. Actually, after going through the book from cover to cover (several times), I thought it would equally apply to either crowd because a lot of what he writes about and explains can be applied to both - all three if you want to put film into its own category.
The book is filled with great info to get good photos, various techniques depending on the subject are explained in detail and all accompanied by helpful little hints and tricks that obviously come from someone who has really taken the time to explain in detail how he makes his images.
I've looked at and borrowed a few other u/w photography books and quite honestly, I found Martin's book to be the most complete and the most helpful. It really is the only u/w photography book, in terms of equipment and technique, you'll likely ever need.
The only other book I'd suggest is one of those coffee-table books filled with amazing photos for additional inspiration (ideas on what to shoot and how to go about doing it - or seeing what's already been done).
Hope that helps. By the way, I have no connection to the author or book other than being a very satisfied customer.
Lee