The accident happened during bad weather, as strong waves caused the boat to overturn.
The speedboat was carrying Indian tourists when it overturned near Phu Quoc Island in the Gulf of Thailand on Saturday.
Local media said rough weather and high waves caused the accident. Rescue operations began immediately, with nearby tourist boats and the coast guard joining the effort.
Many passengers were trapped inside the boat, making the rescue difficult.
While 21 people were rescued alive, at least 15 lost their lives.
Indian Prime Minister Narendra Modi has expressed deep sorrow over the tragedy.
