The vessel, which was carrying 32 tourists plus four crew, capsized on Thursday afternoon after it was slammed by a wave near a rocky stretch of coast on Koh Samui, a popular island in the Gulf of Thailand.
The accident threw all of the passengers overboard, trapping some under the ship's hull.
The bodies of three women -- from the United Kingdom, Germany and Hong Kong -- were soon located while rescuers battled rough seas to search for 46-year-old Briton, Jason Parnell.
"Rescue workers found his body at 9:25am near the accident site," Lieutenant General Ukhcarawath Sithanaubol, told AFP.
"His body was trapped among the rock. Rescue workers are bringing his body ashore by rubber boat now and we have called off the search operation," he added.
Parnell was travelling his wife who survived the accident, police said.
The second British tourist killed, Monica Cozma, was celebrating her honeymoon with her husband of three weeks, who survived, Britain's Daily Mail newspaper reported.
The two other victims were named by Thai officials as 29-year-old German Kafo Franeiska and 30-year-old Hong Kong national Trunk Laidka.
The boats that ferry tourists around Thailand's popular beaches are notoriously reckless and life vests are often in short supply.
Speedboats are also often overcrowded as companies try to squeeze as much profit from each trip.
Police have charged the Thai captain who drove the tour boat with negligence leading to deaths and injuries, a crime that carries up to 10 years in prison.
Although tourism remains a key source of income for Thailand, the kingdom's reputation as the "Land of Smiles" has suffered in recent years amid a string of deadly bus and boat accidents, high-profile crimes against foreigners and a decade of political instability.
But visitors keep coming.
A record high of nearly 30mn travelled to the kingdom in 2015, a number that has been boosted by a huge upswing in recent years by mainland Chinese tourists.