Authorities had been searching for 46-year-old Jason Parnell, from Leicestershire, since the incident on Thursday, when a huge wave capsized the boat off the coast of the island of Koh Samui.
British woman Monica O’Connor, 28, of Manchester, died in the accident, alongside a German man and a woman from Hong Kong, local authorities said.
The Thai tourist police said a fourth body had been found on Sunday morning but it is understood authorities are waiting for formal identification.
Mr Parnell was celebrating his first wedding anniversary with his wife Puja when the crash happened.
Mrs Parnell and Mrs O’Connor’s husband Tim are both believed to have escaped unharmed.
The O’Connors were reportedly on their honeymoon.
The Angthong Discovery Tour vessel was carrying 32 tourists and four crew members when it overturned after hitting rough waters and strong winds.
The boat’s captain, Sanan Seekakiaw, was taken into police custody following the crash and faces a possible charge of reckless endangerment causing death, which comes with a prison sentence of up to 10 years.
Mr Seekakiaw said he had asked all tourists to wear a life vest but that some had taken them off during the journey.
Travel agent Amm Pontfuk, who has worked with Angthong Discovery for a number of years, said the boat had not left shore in the days preceding the accident due to rough conditions.
She told the Press Association: “This company is the number one for my travel agency, I have sent the manager customers for years, I have known him a long time.
“He is very concerned and professional, normally in bad weather he doesn’t go out - he did not go out for three days already - and yesterday he thought the weather was OK and that was why he went out.
“The wind blew very, very strong and it made the boat go under the waves and flip.”