Mizu ni Sumu Hana

From ShoujoMagic: It happened in the blink of an eye. Rikka's world turned upside down when her school field trip bus flipped and plummeted into a lake. Everyone inside that bus drowned... except for Rikka, but she wasn't saved by anything as simple as luck. When she was released from the hospital, Rikka was then attacked repeatedly in a series of bizarre incidents all involving water... and a sadistic aquatic beauty whose name is -- Rikka!? What does our heroine have that the sadistic Rikka wants back? What was it that saved her from drowning, and who gave it to her and why...?