The man, named by Phuket police as Kan Chgn, 28, was on a holiday with his bride-to-be at Nai Harn, a popular southern beach.
The couple were planning to wed soon after returning from their brief Phuket holiday.
Colonel Boonlert On-Kang, of Chalong Police Station, which oversees Nai Harn, said the couple were staying at the five-star Royal Phuket Yacht Club.
''We have been told that the couple had breakfast then the young man decided to go for a swim,'' Colonel Boonlert said.
''His friend stayed in the room a little longer and when she joined him at the pool, she found him floating, without moving.''
The woman called resort staff. The man brought up his breakfast but failed to respond to attempts to revive him.
The couple had arrived on Phuket on Friday and were due to fly out today.
The man's body was taken to Vachira Phuket Hospital.
A spokesperson at the Chinese Consulate in Had Yai said today Kan Chgn's parents were flying to Phuket tomorrow night.
The parents would take their son's body home with them on Thursday.