From publisher blurb:
The azure abyss, the underdeep, the inky wastes, and the midnight realm are but a few names afforded to those depths considered too deep by the common races of the Cerulean Seas. Though these pitch-black depths comprise the vast majority of the endless sea, these realms have been labeled by most as unfathomable reaches. This tome intends to shed some light on these mysterious waters, the creatures that live there, and the reasons why even the bravest of the surface realm fear diving too far down.
This book expands on the material presented in the Cerulean Seas Campaign Setting, bringing you to a new realm of aquatic adventure!