The Cessna 208, operated by AVIC Joy General Aviation, was carrying 10 people from Shanghai's Jinshan district to Zhoushan, an island in the eastern province of Zhejiang, the district government said in a statement.
One survivor is seriously hurt and is undergoing surgery, while four others sustained lighter injuries, the government said.
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The cause of the crash is under investigation, it added.
Xinhua news agency said the airline is the first seaplane operator in eastern China approved to carry passengers.
An airline official, reached by telephone, declined to comment.
AVIC Joy General Aviation is an affiliate of state-owned Aviation Industry Corp of China.
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