Ronald MacNeil, 55, failed to ensure that there was a standby frogman poised to help when Graeme Mackie, 31, entered the Forth 600 metres off Methil Harbour, Fife.
The father-of-one, from Tranent, got into difficulties and resurfaced in distress before sinking unconscious to the river bed on June 11, 2011.
He was eventually recovered and airlifted to Edinburgh Royal Infirmary, where he was pronounced dead.
MacNeil, of Leven, Fife, master of the Rob Roy based at Methil Docks, 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.
However, Louise 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, in consequence of which Mr Mackie drowned.
She said it was “a serious breach”.
The court was told the Rob Roy had been involved in electrofishing, a practice where a generator on the ship sends charges to electrodes trailed on to the bed, shocking shellfish to rise up and then be collected.
However, the depute 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 of 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’.”
Miss Beattie said that Mr Mackie entered the water at 2.24am. She said that MacNeil had checked Mr Mackie’s diving equipment and was satisfied everything was fine.
After entering the water, Mr Mackie resurfaced about “ten to 15 seconds later in distress”.
Ms Beattie said: “He was shouting for help. 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.”
Rescue divers later found Mr Mackie “lying on his back, on the riverbed”, the court heard.
Sheriff Craig McSherry deferred sentence until July 29 for reports. MacNeil could be jailed for a maximum of two years under health and safety, and diving and work regulations.
A spokesman for Mr Mackie’s family said after yesterday’s case: “We note the plea which is an admission, finally, of guilt and responsibility for what happened.
“This is a difficult and emotional time for all of us and we ask that our privacy be respected at this stage.”