The speedboat was carrying 32 Indian tourists and four crew members when it overturned on Saturday afternoon shortly after leaving Hon May Rut Ngoai Island, which is near Phu Quoc, Vietnam's largest island, authorities were quoted as saying.
"The boat had not even gone half a kilometre when it just tipped over," Ashish Kumar, an Indian who witnessed the accident, told The Associated Press over the phone. "We screamed, 'Help! Help!'"
He said nearby boats immediately rushed to the rescue. "But by then it was too late," he said.
Kumar said that there was no emergency medical care available at the shore when survivors were brought back.
He said that at least some of the passengers were on a company tour organised by their employer, which makes smartphones and other electronics.
Three of his friends were on the boat. "Two have died, and the other, I was told, is critical," he said.
India's Lava International, a smartphone and consumer electronics manufacturer, confirmed that some of its employees and channel partners were among those involved in the accident.
"Our immediate priority is to extend all possible support to those affected and their families," it said.
VN Express news site quoted witnesses as saying some of the passenger were trapped inside the boat. Footage on Vietnamese TV showed rough seas and strong winds as rescue teams threw life buoys to people in the water. Jet skis ferried survivors back to shore while people on the beach provided first aid.
Twenty-one people were rescued, and all the dead were recovered, authorities said. The injured were taken to hospitals.
Vietnamese Prime Minister Le Minh Hung ordered an investigation and directed authorities to hold those responsible accountable. He also asked officials to review waterway and maritime safety in the area where the accident happened.
Indian Prime Minister Narendra Modi expressed condolences over the accident and wrote on social media that the Indian Embassy was providing all possible assistance.
Many of those on board are believed to be from India's southern Telangana and Tamil Nadu states.
Authorities in Telangana have set up a control room to coordinate assistance and provide information to their families. The Tamil Nadu chief minister wrote on social media that he had urged Indian authorities to provide assistance to the victims and ensure that the bodies of those killed are brought back as soon as possible.
Phu Quoc in the Gulf of Thailand is one of Vietnam's most popular beach destinations. Hon May Rut island is about 10km south of Phu Quoc. They're known for their white-sand beaches and clear waters and draw millions of domestic and foreign tourists each year.
