Little Mermaid by Jerry Pinkney

Melody, the littlest sea princess, is not content just to sing in the choir of mermaids like her sisters. She is an explorer who wonders about what lies above the water's surface . . . especially the young girl she has spied from a distance. To meet her requires a terrible sacrifice: she trades her beautiful voice for a potion that gives her legs, so that she may live on land instead. It seems like a dream come true at first. But when trouble stirs beneath the ocean, Melody faces another impossible choice -- stay with her friend, or reclaim her true identity and save her family.