Serafina is the princess of Miromara, a merfolk kingdom, and is now old enough to complete the ceremony that will prove she is the next ruler. But during the ceremony, an attack is launched against her mother, the queen. Sera flees the kingdom with her best friend, Neela, a princess from the kingdom of Matali, and the two begin a journey to discover who launched the attack and to prevent war between all the mer kingdoms. But there's a greater evil at work here, and they'll need more help if they want to save everyone.
If you don't like puns and silly jokes, this isn't the right book for you. The world of Deep Blue is built on puns and jokes. The money is currensea. Mermen have merlfriends. Transparensea pearls make you temporarily invisible. The girls meet an old lady with a ton of catfish. One of the main girls has a seeing-eye fish. Luckily, I love puns and jokes. The world-building is just fun and silly. But it is also diverse. The mer kingdoms have different cultures that are reflected in things like clothing and traditions. Merpeople have the tails of different types of fish and are different races.
What I loved about the book is that it's mainly about Sera and Neela journeying together, first to escape and then to try to save everyone by teaming up with other mermaids to fulfill a prophecy to stop a great evil. There's a bit of romance at the start with Sera and her betrothed (Neela's cousin), but that pretty much disappears once the attack is launched against Sera's mother. The other mermaids don't come into the story until the last third, so not as much time is spent fleshing them out as Sera and Neela. What we see of each is still fairly distinct, I just wanted more. But it is the first in a series, so there is definitely room for development.
Deep Blue was just fun. It's a story about a group of girls with different personalities and backgrounds teaming up to save the world. And that is something I always love reading.