Mahou no Mako-chan

Mako is a young siren and daughter to the King of the Sea. In a stormy day she rescues the young sporto Akira from a wrecked ship, but ends up falling in love with him. So, with help from the Witch of the Sea, she's transformed into a human, and becomes a highschool student who helps everyone with her strong sense of love and justice and her magical pendant, the "Tear of the Mermaid"
(Source: Anime News Network)