James McConnell, 69, landed in a reservoir after his skydive near the Thai resort town of Pattaya on Thursday went seriously wrong.
Eyewitnesses told staff at the skydiving centre that he had rolled onto his back and started spinning in mid-air shortly after he exited the aircraft with a group of others.
Harry Harrison, director of flight operations at Thai Sky Adventures, said he believed it was “beyond a shadow of a doubt” that Mr McConnell had experienced a heart attack or a stroke.
Although he managed to deploy his reserve parachute, he appeared unable to steer it as he fell towards the ground. He missed the designated landing spot and landed in water.
“There is no doubt in anybody’s mind that there was a medical issue which is the cause of this incident,” Mr Harrison told the BBC.
“He was either unconscious or dead at the time he landed. We believe he was not breathing at the time he landed, because we got to him within three to four minutes…[and] there was no water in his lungs.”
He added that Mr McConnell, who is understood to be from Clydebank, was an experienced skydiver who was a “regular” at the centre.
A spokesman for the local police force told one Thai news outlet that officers were still investigating whether the pensioner had died “from the jumping or from drowning”.
In a post on Facebook, Mr McConnell’s son Mark McConnell wrote: “It is with a heavy heart that I have to tell my Facebook friends and family that my dad Jim McConnell has sadly passed away.
“As soon as I can confirm what happened I will let you know but for now, the family and I would appreciate privacy at this difficult time! Blue skies to you all xxx.
“All of you who knew my dad will know that he went the way he wanted and no-one should feel sad, fly free my hero xx.”