Jinwoo Park, 21, stopped and sank below the water while swimming to a pontoon on the evening of November 24, 2022.
In findings released today, Coroner Andrew Schirnack said Mr Park had been travelling in New Zealand for about two months before his death.
A few hours after arriving in the township with a South Korean friend, he suggested they go for a swim on the lakefront.
After his friend swam out to the pontoon about 50m from shore, Mr Park followed.
The friend reported Mr Park suddenly stopped swimming about 5m from the pontoon, began "waving his hands to the side" and disappeared below the surface.
His friend called out for help before jumping into the water to search for Mr Park.
Two members of the public came to help, and although one found Mr Park on the lake floor, he could not be retrieved.
His body was recovered by the police dive squad the next day.
The friend told police Mr Park did not show any signs of pain or fear before sinking below the surface, Coroner Schirnack said.
A pathologist concluded there was a "strong possibility" Mr Park developed a sudden, abnormal heart rhythm, "resulting in insufficient blood being pumped by the heart".
That would cause an inability to continue swimming or maintain buoyancy.
The friend said he did not initially realise Mr Park was "in trouble" when he stopped swimming.
They had served in the South Korean navy together, so he knew he was a good swimmer capable of swimming 100m, the friend said.
He was also a regular gym-goer and led a healthy lifestyle.
Coroner Schirnack said he asked Drowning Prevention Aotearoa to provide an analysis of the incident.
The non-profit organisation had made three safety recommendations: hazard assessments for swimming pontoons, safety signage and the availability of rescue equipment.
Although the recommendations were "sound", he saw no clear link between the death and the absence of those safety features at the location, he said.
Mr Park had been young, fit, liked the water and was a competent swimmer, and would probably have gone swimming to a depth over his head regardless of whether there was a pontoon or safety signage.
