Adipong Tapee, a local police officer, said of the 42 passengers injured, 39 were taken to hospital on the mainland in Surat Thani province, and they included four Britons, four Australians and four Singaporeans.
He also said among the injured were Irish, Norwegian, Malaysian and Thai tourists.
However, police said they could not confirm the exact number of people on the boats and the boat drivers disappeared immediately after the accident.
The cause for the crash was still under investigation, but authorities said it had been raining when the boats collided.
The Koh Pha-Ngan island, which is about 1,000km south of the capital Bangkok, is well renowned among party-goers from across the world.