Local police said the victim, identified as Richard Ernest Spraggs, 59, fell into the sea at Chalong Pier in the Phuket provincial seat, apparently hitting his head on another boat and that he probably died from drowning.
Police found a wound on his head but no sign of being hit anywhere else on his body.
According to initial investigation, Mr Spraggs was the owner of the yacht and sailed to many countries, visiting Phuket every year for the past six years.
Police said the death was believed to be caused by drowning, and that it was an accident.
At around 3am, Mr Spraggs used an inflatable boat to pick up a friend at Chalong Pier.
After he returned to his vessel, the waves drove a larger boat moored nearby into his yacht.
Trying to push the other boat away, he lost his balance and fell into the water, hitting his head on the way down.
Police said he may have been knocked unconscious and drowned.