The Thanh Phong tourist boat, codenamed QN-6018, was anchored near Reu Island in Ha Long and the weather was reported to be normal, news website VnExpress reported.
Water then flowed into the boat, causing it to sink. Three crew members jumped out of the boat and swam to safety.
Local authorities said the boat may have hit rocks lying scattered across the edge of the island when the tide ebbed and holes developed.
Rescuers would soon tow the boat ashore, they said.
