Police have not yet officially determined the cause of a fire on a speedboat heading from Chumphon to Koh Tao on Wednesday.
Pak Nam Chumphon police station chief, Pol Col Sathit Kongniamin, said officers were still gathering and piecing together evidence.
Although the fire appears to have been the result of an engine explosion, it remains unclear what caused it, he said.
The incident, about 5 kilometres off the Chumphon coast, injured 20 people including 18 passengers, the driver and a crew member.
It happened around 2.30pm after the speedboat belonging to Lomprayah High Speed Ferries Co departed from a pier in Muang district.
The vessel initially used two engines; 20 minutes later the driver turned on the third and there was an explosion, the police chief said.
Most passengers jumped into the sea and were slightly injured; four others suffered burns.
Police will work with the Marine Department to bring the vessel to shore. An expert from Surat Thani has been called in to verify the explosion's cause.