Summary from Goodreads:
Deep in the ocean, in a world not so different from our own, live the merpeople. Their communities are spread throughout the oceans, seas, and freshwaters all over the globe.
When Serafina, a mermaid of the Mediterranean Sea, awakens on the morning of her betrothal, her biggest worry should be winning the love of handsome Prince Mahdi. And yet Sera finds herself haunted by strange dreams that foretell the return of an ancient evil. Her dark premonitions are confirmed when an assassin's arrow poisons Sera's mother. Now, Serafina must embark on a quest to find the assassin's master and prevent a war between the Mer nations. Led only by her shadowy dreams, Sera searches for five other mermaid heroines who are scattered across the six seas. Together, they will form an unbreakable bond of sisterhood and uncover a conspiracy that threatens their world's very existence.
At the beginning, I felt quite overwhelmed with the info-dumping and terms used by the characters to build the Waterfire Saga world. I could tell explanations/translations were added within a character's speech for the reader's benefit but the dialogue between the characters felt stilted and awkward. This however eased as the book went on.
Creative and imaginative concepts about the way of life, food, cosmetics and fashion made me chuckle throughout the book. Common sayings were twisted to incorporate the mermaid's point of view, such as 'from all swims of life' and 'you stick out like sore fins'. Some of the ideas were a bit cartoonish and silly which made me confused whether the overall tone of the book was meant to be more serious/epic or light-hearted. The scale was frequently tipped from one side to the other.
In terms of characters, only the surface of each were tapped into since there were so many introduced within the 300 odd pages. The destinies of the main characters shaped them rather than their traits or back-story, hence I was sitting on the sidelines observing rather than being invested in them.
The incorporation of Atlantis city provided a cool foundation for the world's history. The politics in the story was rich, though sometimes boring and hard to follow with all the exotic names and titles. Overall, Deep Blue did a good job of setting up the series.
Note: I received an advanced reader's copy from Hachette Children's Books via NetGalley to review. This review is my honest opinion.