Aquamarine - Alice Hoffman

Hailey and Claire are spending their last summer together when they discover something at the bottom of the murky pool at the Capri Beach Club. There in the depths is a mysterious and beautiful creature with a sharp tongue and a broken heart: a mermaid named Aquamarine who has left her six sisters to search for love on land. Now, as this mythological yet very real being starts to fade in the burning August sun, a rescue is begun.
On the edge of growing up, during a summer that is the hottest on record, Hailey and Claire are discovering that life can take an unpredictable course, friendship is forever, and magic can be found in the most unexpected places.