This is a simple book that describes a range of magical environmental hazards that characters could encounter while in, or on, sizeable bodies of water.
The hazards mostly range from CR1 to 5 (a few are variable), and are described with simple rules and an explanation of their origin. Many are drifting clouds of magical energy that deliver various types of damage, while others are static forms of sea life, such as coral or sponges. A few are rather more deliberate creations, and the book rounds out with a 3rd level spell for creating one of them.
Interestingly, pretty well all of the hazards outlined here could work just as well above water with little, if any, modification. A cloud of magical energy, for example, can surely float through the air as easily as through the sea. Nonetheless, the aquatic theme adds a nice touch and makes some of the hazards seem a little more unique than they might otherwise - making the aquatic environment feel different to that on dry land.
The quality of the production is actually pretty good for a $1 book, and overall, I'd recommend this as worth a look.
[4 of 5 Stars!]