This book is designed to help you make the seas in your campaign world more than blue patches on the map that the party may occasionally travel across in a boat. Most adventurers, after all, are accustomed to breathing air and walking on land, and to do anything else (bar a bit of splashing around in the shallows) is going require magic or technology to accomplish. But what about those intelligent - perhaps even sentient - creatures that live there? Might you run adventures involving them? Or ever a campaign in which they, rather than the more conventional land-dwellers, are the party?
The first chapter, Ocean Life, looks at general aspects of life under the sea, starting with the sort of things that are familiar if you're a SCUBA-diver - depth, pressure, light levels, the chill of the depths, and just thinking in three dimensions in a way the land-based rarely do (except, perhaps, if you can fly). In general oceans are dark, cold, dangerous places and most residents therein are likely to want to eat you. Here we also learn about algal blooms and the problems they can pose, buoyancy - which controls how well you can hold your position at the depth you want to be at - currents, light level and even why it's not a good idea to drink seawater. There's a word about storms, and a note about looking after your spellbooks - regular land-dwellers' ones are likely to fall apart when submerged. Aquatic magic users make theirs out of coral and bone and similar more durable substances, but they tend to be large, cumbersome and heavy - especially if you try to take one onto dry land. Tides and tsunamis can alter the environment, and even fantasy worlds can suffer pollution!
There are some general notes on aquatic plants and animals, but these are covered more thoroughly in the next three chapters. These look at coastal waters, the open sea, and deep water. In each chapter there are notes on more than just plants and animals with hazards and a selection of monsters presented. The Open Sea chapter also talks about floating cities. The monsters are quite innovative and include undead as well as monstrous variants of actual sea creatures and some really strange things as well. Did you know that some seafarers claim that you shouldn't rescue anyone who falls overboard because the sea takes who it pleases. Only some people fall in by accident (or because someone shoved them in) when the sea didn't want them at all, but don't get rescued by superstitious sailors... and become the Unwanted, roaming the oceans seeking to kill the living and sink their ships. Suggestions for adventures or even whole campaigns are scattered throughout; and some of the monsters are sentient and there are notes on using them as NPCs or even player-characters.
Assuming that you'll mostly be dealing with land-dwellers exploring the ocean, Chapter 6: Equipment sets out to enable them to find the gear they'll need to survive, if not thrive, underwater. It also looks at what those who live underwater make and use for themselves, which may be of interest to enterprising explorers who realise that things made in an environment may well be best suited to that environment. There's a note that most underwater communities operate by barter rather than using money, so explorers need to come prepared to trade for what they need. There are some interesting ingredients for those who practise alchemy, including some particularly potent poisons.
Finally, Chapter 6: Spells attends to all your magical requirements. It's the main way in which surface dwellers survive underwater, or for that matter sea-dwellers survive on land. A range of spells for bards, clerics, druids, rangers, and wizards are provided, with spell lists and full details of each spell provided. There's even a spell to allow a bard to perform underwater! Appendices provide random encounter tables for all parts of the ocean.
A useful work if you think your adventures might be heading out to and particularly under the sea. Perhaps the next ocean voyage won't be plain sailing, with the characters having to survive in and under the open ocean until they are rescued or can make their way ashore. Ideas and concepts will work whatever ruleset you are using, even if you need to tweak the mechanics a bit!