Ronald MacNeil, 55, failed to ensure there was a standby frogman poised to help when Graeme Mackie, 31, entered the Firth of Forth 600 metres off Methil Harbour, Fife.
Mr Mackie, from Tranent, East Lothian, got into difficulties and resurfaced in distress before sinking unconscious to the river bed on 11 June 2011.
He was eventually recovered and airlifted to Edinburgh Royal Infirmary where he was later pronounced dead.
MacNeil, of Leven, Fife, master of the “Rob Roy” based at Methil Docks, Fife, had been due to face trial at Dunfermline Sheriff Court accused of being responsible for a series of health and safety failings that led to the tragedy.
Loiuse Beattie, prosecuting, accepted a plea to single failing – not having a standby diver who could have gone to Mr Mackie’s aid in an emergency. She said it was “a serious breach”.
The court was told the Rob Roy had been involved in electro-fishing, a practice in which a generator on the ship sends charges to electrodes trailed on to the bed, shocking shellfish to rise to the surface of the sandy bottom, where they could be collected.
However, the deputy fiscal said there was no evidence this was a factor in Mr Mackie’s death, which was caused by drowning.
The court heard that Mr Mackie had previously been employed as a welder and had undertaken “an intensive underwater dive course”.
Ms Beattie, specialist health and safety prosecutor at the Crown Office, said that Mr Mackie’s ambition was to eventually become an underwater welder, and that he had advertised himself as a trainee shellfish diver online, prompting MacNeil, a fisherman with many years’ experience, to get in touch.
She said: “On the morning of the dive, Mr Mackie woke his partner up to say goodbye. His partner was nervous but excited for him. Mr Mackie considered the dive as ‘getting a start’.”
The depute fiscal said both Mr Mackie’s father and brother were not aware that he had any previous commercial diving experience.
On the day of the fatality, Miss Beattie said that Mr Mackie entered the water at 2:24pm.
She said: “All of Mr Mackie’s dive training involved scuba diving. He was connected to the surface by an umbilical line. He was wearing his own dry-suit. There was no stand-by diver on the boat.”
After entering the water, Mr Mackie re-surfaced about “10 to 15 seconds later in distress”.
Ms Beattie said: “The accused shouted to him to drop his weights. The accused entered the water and swam towards the last sighting of Mr Mackie but was unable to locate him.”
A short while afterwards, rescue divers were called to the scene and managed to locate the deceased.
He was “lying on his back, on the riverbed and the mouth piece was not in his mouth”, the court heard.
Sheriff Craig McSherry deferred sentence until 29 July for reports.