Deep Blue, by Jennifer DonnellyRead about the other entries on the list.
Serefina is a mermaid who will some day become the ruler of her people. Sounds great, except for the dreams she’s been having, about an ancient evil returning to cause chaos. After her mother is shot with an arrow by an assassin, Serefina embarks on a quest to unite with mermaids across the mer nations, to prevent their world from falling apart. Apparently the mer world is a rougher place than The Little Mermaid led us to believe.