The boats were ferrying tourists to and from Pha-ngan island, site of "full moon" parties that attract thousands of young, mostly Western tourists each month.
Police Lt. Pongkajorn Sukrasang said the boats collided and overturned around midnight Saturday, throwing the passengers into the rough sea during a rainstorm just off the island.
Two Thai brothers were listed as missing but were reportedly sighted later on the island, Pongkajorn said.
Thirteen of the injured remained hospitalized Sunday afternoon, he said.
The passengers included tourists from Britain, Australia, Singapore, Ireland, Norway and Malaysia, he said.
"They're lucky because all were wearing lifesaving vests.
Most of the injuries were minor," the police officer said.
The monthly parties on the island, 1,000 km south of Bangkok, are rave-type gatherings fueled by drugs and alcohol and have become associated with crimes such as rape and theft because of their freewheeling atmosphere.
Police said the accident had little effect on the party. A number of apparently drunk foreigners were lying on the beach Sunday morning near where the boats had collided.
