This book is a steal at the current price, though I admit the price used to be higher and that kept me from checking it out. Now that I have it, I intend to buy the print edition. Very short but here's the pros and cons of the book:
Pros: a fantastic resource for an undersea campaign environment and very comprehensive at that; gorgeous illustrations, the nicest 3PP Pathfinder book I have yet seen; full of useful and interesting content; provides three dimensional rules for undersea combat.
Cons: Very specific to undersea adventuring, so of limited use if that's not your thing; the undersea combat rules look a bit complex.
Style and Presentation A+
Content: A+ for what it sets out to do; a C if you are looking to mine for ideas in on undersea campaigns.
General Value: A+
[5 of 5 Stars!]