In fact, this book is pretty much up to par with the first one, if not better. Anna is a bit more mature about the way she handles things, and she's not so traumatized by her best friend's death. Granted, she has all sorts of other issues she has to deal with, but I feel like she handled things a bit better in this book. Plus, there was very little (maybe even no) high school drama, which sort of distracted from the whole underwater plot that was going on.
Plus, we get a bit more action in this book. Of Poseidon was all about the mystery of where Emma came from and discovering her heritage/abilities. This book has more conflict. First we get Nalia running away from Grom (who she is convinced is dead and that she's wanted for murder), having tied of Reyna and kidnapped Emma. After that, we have a political conflict involving Nalia's return and the position of all the royals, really, and their right to rule. Finally, we have the whole humans discovering the Syrena problem. All in all, it keeps the book moving.
I just have one major complaint. It's sort of established that the two underwater kingdoms (Triton and Poseidon) are having conflicts. But when Nalia comes back there's not the conflict between them like the synopses suggests. Instead, a slimy political maneuver sort of puts all the royals on trial for their right to rule, and you can't really tell where one kingdom ends and another begins. It didn't really make sense, especially if they really are two kingdoms. Maybe they didn't explain enough about the underwater society in the first book, but half of it is from Galen's POV, so that's not really an excuse.
All in all, this is a great 3.5 stars for me. It's also one of the better mermaid/sea creature type book I've read recently. So if you're looking for a great beach read, you really should check this out. I'll admit, I kind of wish I had waited to read this by the ocean. Then I wouldn't have to be so jealous reading about sand between their toes and swimming in the ocean.