Ano Koe ga Kikoeru Umi

Midori Urakaji was an ordinary high schooler.
Recently paired with a freshman transfer student named Ruri Akitsuki, they both trained for the swimming club. While at the beach Midori discovered a hairpin in the shape of a little dolphin. Midori clipped it on... and felt a transformation.
Ruri soon found out about Midori's change. Rumors began to swirl in swim club.
It was the start of a double life that would involve Midori, Ruri, and friend Arashi Nobun.

[from DLsite English]