Koh Samui district chief Paiboon Ohmak said the accident happened at 4:30 pm and officials rushed out to rescue the tourists.
Paiboon said the Angthong Discovery 3 speedboat was returning from the Angthong islands and capsized near the Khakhi cape in Tambon Angthong, Koh Samui.
The boat was hit by a strong wave and capsized, four crew members of the boat testified. They were safe.
The accident saw 28 passengers injured. They are 15 foreign men, 12 foreign women and a Thai woman.