Mizu ni Sumu Hana

In a regular school travel, the bus Rika Nikaidou took fell into a lake, drowning everyone on board except her. She vaguely remembered a person fed an item into her mouth when she was suffocating in water, but that was all she could recall. After her recovery, she found herself more and more agile in water, capable to stay underwater over ten minutes, while feeling less and less comfortable on land. One day, when she was practicing in school's pool, a beautiful yet completely stranger girl suddenly appeared in water, trying to to "reclaim the 'seed'" from Rika with force. (Source: ANN)