By Thai Visa
The tourist was not named but is aged about 40.
He was one of eight tourists and crew who had set out from the pier at Koh Taen in Taling Ngam sub-district on Monday afternoon.
The man had been sitting at the head of the speedboat without a life jacket when a wave hit, causing him to fall into the sea.
Koh Samui district chief Kittiphop Rotdon said police had mobilised a search after the alarm was raised at around 6pm.
But as night fell, darkness hampered the search operation.
The search with Royal Thai Navy help, including a helicopter and divers, was due to resume at daybreak.
It was a case of negligence as tourists on boats should wear life jackets.
The other tourists on the boat and the crew are helping police with their inquiries, they said.