At a Department of Transport hearing in Southampton, Adrian Rayson, a crew member, said: 'He did not jump, nor did he fall. The dive appeared to be a premeditated act.'
William Vincent, 47, a carpenter from Bath, appeared depressed before diving overboard 450 miles off the West African coast during the British Steel Challenge, an eight-month amateur race in which 10 yachts circumnavigated the world.
He had been one of 13 crew members on Heath Insured, which sailed into Southampton on Thursday, the last yacht to finish the race.
Samantha Brewster, 24, of Woodbridge, Suffolk, said she saw Mr Vincent out of the corner of her eye when he went to dive off.
'I turned round to look at him. He had his left foot on top of the pushpit and was stepping up. I yelled 'Bill'. At the same time he dived over the stern and swam away,' she said.
He looked back at the yacht, but he appeared to make no effort to attract the crew's attention, Mr Rayson told the inquiry.
Adrian Donovan, 35, the captain, said at the hearing attended by Mr Vincent's wife Pauline, a teacher, that he heard Miss Brewster scream that a man had gone overboard. He was on deck in seconds, but although buoys were used to mark the spot, the crew lost sight of Mr Vincent in the swell.
Lookouts were posted and a search continued through the night but he was not found and, eventually, the crew held a short service in the cockpit before resuming course.
Mr Vincent, who had two teenage sons, had appeared preoccupied since leaving Cape Town on 17 April on the last leg of the 28,000-mile race, Mr Rayson, of Henley, Oxfordshire, said.
'Since leaving Cape Town Bill Vincent had seemed prepossessed with a personal problem and appeared listless and depressed.'
But in the two days before he went overboard on the evening of 29 April, he seemed happier. 'I did not have a hint of his apparent desperate state of mind. His diving overboard was a tremendous shock,' Mr Rayson said.
Michael McDonnell, the marine superintendent holding the inquiry, said the crew had made every effort to try to rescue Mr Vincent. He made a formal finding that Mr Vincent was lost at sea, believed drowned.