A man at "high risk of suicide" was found unconscious in a bath on a mental health ward, an inquest has heard.
Alyn Mason, 28, deliberately crashed his car on Christmas Day 2016 and was admitted to Ash Ward at Oakley Court psychiatric unit in Luton the next day.
He was found face down in the water on 27 December and died in hospital.
Recording a narrative verdict, the coroner said Mr Mason, who was diabetic, most likely fell unconscious because of hyperglycaemia.
In a statement Mr Mason's parents said they were "disappointed that no answers have been provided for his hypothermic state with a body temperature of 31.6C, when Alyn left Ash Ward".
"Ash Ward have advised us that they have made significant changes that we hope this will prevent any similar events in the future," the added.
A psychiatrist told the inquest Mr Mason wanted to leave the unit, but she insisted he stay and be observed at least every 15 minutes, because he was at "high risk of suicide" after a "suspected psychotic episode".
The Court House in Ampthill was told that Mr Mason, of Regent Close, Bedford, died from drowning and was also suffering from hyperthermia.
Coroner Ian Pears said his unconsciousness was "unlikely to have been the result of self-drowning or a fall".
He said hyperglycaemia - or a high blood sugar level - was the most likely cause though it was not known why he failed to react to the symptoms.
Mr Pears said he accepted observations by staff every 15 minutes was "appropriate" and had been carried out.
In February, the health watchdog reported Mr Mason was one of four patients to have died "unexpectedly" on Ash Ward in eight months.
The Care Quality Commission said Ash Ward had reduced its number of beds from 27 to 19. The Royal College of Psychiatrists' recommended maximum number of beds is 16.
At the time, East London NHS Foundation Trust (ELFT), which runs the unit said it had been "focussed on learning lessons from the incidents".