The collision took place about 800 metres offshore from Koh Larn's Tawaen beach.
Ten of the injured tourists were sent to Bangkok Hospital Pattaya and eight to Pattaya Memorial Hospital.
The two who were seriously injured were identified as 51-year-old Myung Be Kin, whose legs were nearly severed in the crash, and 63-year-old Gihwa Song, who lost one leg entirely.
The two drivers of the speedboats fled the scene of the crash and remain at large. Police are carrying out a manhunt to find them.
Police said a total of 28 South Korean tourists were travelling on the two speedboats to the island when the boats collided.
The passengers were wearing life jackets and were rescued from the sea.
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