Sihanoukville police chief Gen Chuon Narin said the boat overturned after being hit by strong waves today, about 18 kilometers from the coast.
He said the Chinese had rented the boat to visit Koh Rong island even though authorities had banned all sea traffic including fishing boats because of bad weather.
Koh Rong island is regarded as the most beautiful of Cambodian islands because of its white sand beaches and virgin forests.