Officials believe the pair may have been swept as far as 60 miles away from the point where the diving boat, called the Reggae Queen, sank.
Ms Young, an artist and teacher, has been living in Thailand since 2007, according to a social media account linked to her.
She had been traveling with friends Jo and Phillip Degregorio, and their two children Oska and Daisy, who are also British.
The boat was carrying 18 people when it encountered rough waves and began to take on water about four nautical miles from the Surin Island chain. It made a mayday call shortly before 9am and sank shortly after, according to officials.
Several passengers, including Ms Young’s four travel companions, were pulled aboard a passing fishing boat and taken to safety while Ms Young and Ms Chaichana were swept away.
