Okay, so a little while ago I bought Cerulean Seas from Alluria Publishing. To be honest, when I first heard about this product I sort of blew it off, visions of Ariel the Little Mermaid began swimming around in my head. But later on I was in search of more campaign world settings to devour and gave it another look. One of the things I decided for my new campaign world is that the undersea and underground regions will be as richly peopled as the dry land above ground. So I decided that I should check out Cerulean Seas.
Cerulean Seas is superb. Rather than publishing a Pathfinder add-on sourcebook which allows DMs and players to extend their campaigns into occasional underwater jaunts, they stripped Pathfinder down and rebuilt it from the ground up. Cerulean Seas is set on a world where the dry land civilizations were destroyed, leaving only the undersea ones. That puts all the focus on the oceans, although there are a couple amphibious races. The book starts with a good overview of the undersea and coastal environment and their effects on movement, etc. Next are the races: there are twelve races, a couple from Pathfinder but most completely new. The new races provide a great range of interesting options to play but you need to mentally reorient from the standard humans, dwarves, etc. The standard core classes from Pathfinder are included albeit with slight adaptations but there are also three completely new races: the kahuna, mariner, and siren. These are designed to replace the druid, ranger, and bard classes. I think they're all pretty cool.
The game also has its own set of deities, different currency types, a range of completely new armor types, a whole marine bestiary, and a lot of other cool new stuff custom designed for the new setting. I'm really impressed with the care and thoroughness the authors took to really review, research, and rethink to create a fully detailed campaign setting. So if you're considering running an undersea game for a change I do recommend Cerulean Seas.