Patrick Webster, 33, from Seaford in East Sussex, died in the accident by the A259 in East Guildford, near Rye, in the early hours of 30 January.
Mr Webster was driving with his wife and sister when he decided to stop and find somewhere to go to the toilet.
He was found dead in a water-filled ditch after the alarm was raised when he did not return to the car.
"Sadly he chose the darkest possible place in the marshland"
Pc Sean Clarke, an officer with Kent Police, discovered his body lying in the pitch black in bitterly cold conditions.
Pathologist Dr Stuart Barnes told the inquest in Hastings on Wednesday: "I think death would have occurred fairly quickly, possibly in a matter of seconds.
"If he had fallen into the water head first on what was a freezing cold night, he would have got one hell of a shock.
"That, together with lungs full of freshwater, would have been fatal."
Mr Webster's wife Luan said in a statement that she and her sister-in-law started shouting his name when he did not reappear.
East Sussex coroner Alan Craze recorded a verdict of accidental death.
He said: "Mr Webster had been wanting to go to the lavatory for a long time but sadly he chose the darkest possible place in the marshland."
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