William Peace was found dead in July 2017 after he took part in a 45m dive off the coast of Dunbar to find the wreck of a 755-tonne German mine laying submarine, which sank in 1916.
Diving instructor Ashley Roberts, of Huddersfield, had decided that Mr Peace and another pupil should undertake the closed-circuit rebreather dive so he could assess their competency, but when they descended to 13m, he returned to the surface to check on a friend whose dry suit was leaking.
When he got back into the water, he could not find the two pupils, who had continued to the seabed and then encountered difficulties.
Several unsuccessful attempts were made to rescue Mr Peace, before his body was later recovered by police divers using a sonar search.
An investigation by the Health and Safety Executive found Mr Roberts had failed to carry out a suitable assessment of the pupils’ competency before they entered the water and an assessment should have been done at a much shallower depth.
The HSE also said he should have supervised the pupils throughout the dive.
He pleaded guilty to breaching Section 3(1) and Section 37(1) of the Health & Safety at Work etc Act 1974 at Edinburgh Sheriff Court and was fined £2,300.
His company Ash Roberts Technical Limited was dissolved in July 2019.
Alister Wallbank, a HSE specialist diving inspector, said: “This was a highly traumatic incident for all involved and a tragedy for William Peace and his family. Mr Roberts was responsible for the appropriate level of assessment, instruction and supervision.
“The conduct of Mr Roberts undertaking at the pre-dive and assessment stages exposed William Peace and his co-pupil to increased risks to their health and safety than might otherwise have been the case.
“Diving is inherently risky and particularly more so when divers are undergoing training and assessment.
“There are many potential risks and it is ultimately the responsibility of the diving instructor to manage these risks when supervising, training or assessing in what are often dynamic situations.”
He added: “Many diver training courses require an initial assessment dive in order to establish that divers can demonstrate the required pre-requisite competency before progressing to formal training and assessment of more advanced skills and techniques.
“Competency is a combination of skills, knowledge and experience, it is a recognised fact that many previously learned diver skills can fade over time if not routinely or recently practiced.
“It is vitally important that a diving instructor adheres to the training guidance provided by the diving federation under which they are instructing and conduct these initial assessment dives in such a way as to reduce any risks so far as is reasonably practicable.”