Patrick Webster, 33, was found floating face down in a water-filled ditch at Kent Ditch Corner, on the A259 near Rye.
Mr Webster, a petrol pump attendant from Seaford, had left his wife and sister in the car to answer a call of nature on a freezing, pitch black night in January, an inquest at Hastings heard this week.
The group had been visiting a friend in a hospital at Romford, Essex, but had got lost on their way back to Sussex, ending up in Canterbury. It was around 3am when Mr Webster pulled over.
His wife Luan and sister Maryanne raised the alarm when he failed to reappear. They were helped by passing van driver Christopher Rusiecki, who used a torch to search nearby fields and irrigation ditches without success.
Mr Webster's body was found in a ditch, 15 feet away from the car by PC Sean Clarke, of Kent Police.
Pathologist Dr Walter Barnes said Mr Webster could have been unconscious within seconds of falling in the water.
He said: "The sudden shock of the change of temperature could cause this. If he fell head first into freezing cold water in the dark he would have got a hell of a shock."
Luan, who had been married to Patrick for six years, said: "Pat was a little wound up about being lost. He suffered from panic attacks, weak knees and depression but he was in a good mood when he left the car. He was just desperate to go to toilet and told us he could not wait any longer."
Sister Maryanne said: "We were so shocked. We couldn't believe it. Patrick was fine when he left the car.
"He had never learned to swim as a child and when he became an adult considered there was no point."
Coroner Alan Craze, in recording a verdict of accidental death, said: "This is a tragic and unusual death. He stopped on the darkest possible place you could have found on the marshes by a very steep ditch."