The boat started to sink about 100 meters (328 feet) from a pier in Pattaya due to a leak on its floor, police Col. Thammanoon Mankong said.
He said the 27 South Korean tourists were rescued by authorities and crew from nearby boats. One of the tourists was slightly injured and taken to a hospital. The boat's crew members also were rescued.
The boat was returning from Lan Island, a popular getaway for snorkeling and swimming day trips in the Gulf of Thailand, 7 kilometers (4.4 miles) west of Pattaya.
Pattaya, one of the most popular destinations for foreign tourists in Thailand, is 100 kilometers (60 miles) southeast of Bangkok.