The 30-metre (98-ft) boat had 25 people on board when it sank on Thursday, including eight foreigners - four Singaporeans, an Indonesian, a Malaysian, an Indian and a Japanese, it said.
Three people, all Chinese, had been rescued. All eight foreigners died.
Sembcorp Marine Ltd, a Singapore-based shipbuilder, said on Friday the tug boat that its subsidiary, Jurong Marine Services Pte Ltd, had planned to charter sank during a sea trial. Three of the companys employees were on board.
Sembcorp Marine is part of Sembcorp Industries Ltd , an industrial conglomerate with Singapores state investor Temasek Holdings being the biggest shareholder.
Xinhua said the 368-tonne boat was built by Anhui Bengbu Shenzhou Machinery Co. Ltd. in October.