When it come to "Good" books on underwater photography, there is little point is any serious review stating this product is better than that product because each work invariably includes different elements of the overall subject. This is largely because different authors have different experiences on which to draw.
There was a time when I was so very comfortable with my Nikon 35mm cameras in their large aluminium housings I really didn't care that the equipment was out-of-date - all I needed was film. Over time, the results of my endeavours had supported four books of my own, many by other people and I don't know how many articles. One picture was even used by National Geographic as the cover shot for one of their videos!
And then came Digital and I had to change almost everything. My personal conversion proved to be a VERY steep leaning curve and even now I am not quite as comfortable as I used to be as few short years ago. I mention all this because I never used to buy books on underwater photography - not because I knew it all, certainly not! It was because I was plodding along quite happily (and moderately successfully!) - and if it ain't broke don't fix it.
Now I take every opportunity to continue the ongoing education demanded by Digital photography. Not all the books I read are on underwater photography but of those that are, this is only of moderate use. It is not as comprehensive as one might expect, does not contain sufficient information on several separate areas and, it must be said, some of the photographs are not as good as one would expected to find in a book which says this is how it should be done! Nevertheless, there are lessons to be learned so it does provide a moderately useful addition to my personal library.