Recently played a couple of nautical adventures with my local gaming group, and with that in the back of my mind while reading this supplement there are a couple features that stood out to me as being particularly useful.
On the inside front and back covers there are authentic floor plans for two types of medieval sailing vessels, which is a handy feature for DM's setting the scene for shipboard actions. These are scaled for miniature combat as well.
Another useful development in this book is the use of the "crew" monster type, which simplifies ship to ship actions by consolidating multitudes of crew members into one unit for combat purposes, and there are several examples of distinct crew types that are grounded in the "Kingdoms of Legend" setting but are general enough (as is the rest of the book) to fit well into any late medieval/early renaissance campaign.
Also includes a new class type of mariner as well as three unique prestige classes. The late medieval firearms rules seem well developed as well.
As far as negatives go, the artwork did not all download (which was noted by another reviewer as well). Overall, perhaps 90% of the information in this book seems to be generally applicable to any campaign setting in the appropriate era, with just a small amount specific to this particular campaign setting. I think this makes the book more useful to the gaming community rather than less, although some users may be hoping for more fleshing out of the "Kingdoms of Legend" setting than is so far available. Also, while the rules are relatively generic so far as setting is concerned, they very much build upon those of the Pathfinder/3rd edition game system, and may not be especially useful for players using other rule sets.
Altogether, in my opinion, a well written and useful rules supplement for gamers looking to adventure on the high seas using the Pathfinder/3rd edition rules.