Maybe it's just because this didn't start well, but this book felt "off" from the first one. The tone at the beginning is very different, almost environmentalist, and it just doesn't fit a sequel to a Caribbean pirate story that didn't have that tone to start with. So if you're going to glory in the beauty of a sea turtle den (among other things), that won't fit stylistically when you later toss a crocodile in there too (spolier: the crocodile has a good time, the sea turtles don't).
Also, the author is messing with different ways to refer back to the important events of the previous book to catch up those who may have started with this one, such as reliving some of the events in a compressed dream. It's okay, but this is why I have a pet peeve about books that don't stand on their own. Trying to connect these two books which should probably have been one book is messy, even if the author was more creative than usual about trying to tie them together. If you're not going to make your books complete stories on their own, you should probably just admit they don't and you shouldn't bother trying to help someone who is starting with this one.
Unfortunately, this one also doesn't end in a completed manner. I didn't realize there should be more (no placeholders exist for them on Goodreads), so I've violated my Jordan-Martin-Rothfuss policy of not starting a series until it's completed. This is exactly why I have that policy!
Once the rough beginning is over the story is quite good again, very much like the first one. There's action, piracy (of course), and the revealing of yet more dark secrets. I could have sworn the X of the title was modeled after Blackbeard, but apparently he's an actual (different) historical figure.
So it's good; I just wish the story were complete.