Shinkai Shoujo

A year ago, Hikari Tanaka, who went missing in the sea, was rescued from the bottom of the sea by the Deep-Sea King, and came back as a deep-sea girl with the power of deep-sea creatures! Hikari's childhood friend Yamada is happy that she's back, but is puzzled by Hikari's strange behaviors like lighting up and turning transparent. Even so, since he's in love with Hikari, he tries to accept her, but...?!

(Source: Kodansha, translated)