I feel in a basic review mood, so I'm just going to cover the bullet points. Plus since this is a water type book, I get to put cheesy fishy and water metaphors in my review.
Things I liked -
◈ The cover is gorgeous.
◈ The book has likable - although simple and expected- characters. The protagonist is the girl next door, a head of the swimming team, great family, loyal to best friend, crush on the brother. The male protagonist is also likable enough, strong spirit and strong ties. The best friend of Ash, and the brother of Fin, is also a typical type - wanting to swim away and escape the ties of what she is. Characters in this one, while nothing exceptional in terms of being different, fit into their trademark roles well enough.
◈ The relationship is sweet and simple - while I'm not a fan of the strange kiss thing (more about that below), the way they are with each other does get the heart a tug.
◈ Mermaid world!
◈ Cutesy water-inspired sayings like "Holy Crawfish!", "Great Poseidon", "A match made in water." It's like Batman's Robin had a baby with a mermaid.
Things I disliked -
◈ Mermaids in this world are promised to another if they choose to kiss them. Something mystical about their souls mixing or whatever. This is odd because he ends up promised because of mouth-to-mouth rescue. Any kiss can force a union, including forced kisses. I hate that concept. It cheapens life-long soul mates to me - there should be intent as well.
◈ Strange point of view play. The book is written in first person point of you, with "I", but it is written in two people's points of view. It should have been he/she since there are two narrators. In addition to this, each chapter is divided into the other person's role. It will say "Ash" or "Fin." This mechanical way of storytelling may work on paper in theory, but it makes the story a little mechanical/choppy.
◈ Not much happens besides from Fin's point of view. The romance is supposed to be a major part but that doesn't swim to the top until toward the end. Too much time to get there as well. It would have been better if more time were focused on Fin since that's where the action played out.
◈ A little juvenile with the writing style. It's aimed for YA but I think it reads more for middle school type.
Overall it's not a sinker, but it's not a swimmer. Mermaids are fascinating to read about, and the characters work okay, but there's a lot of changes needed to bring this book up to par.