"In my view, the death was unnecessary and avoidable," Mr Shortland said.
He made a "personal recommendation" to all divers, saying they should be confident and competent in their equipment, which should be of the correct size, and have good quality air in their cylinders.
Those who were not comfortable should not dive.
His other recommendations, which he said were not binding on the Pacific island nation, included that divers should remain in pairs or in groups and have a plan if they separate, and dive guides should remain side-by-side with those they were escorting.
"The recreational diving activities in Vanuatu are world famous and reported to be some of the best diving in the world. These recommendations can only but strengthen a thriving industry," Mr Shortland said.
The police investigation, allowed by local authorities, found many shortcomings in Aquamarine's operation.
"The police report concludes that some of the dive equipment used by Dr Osunsade directly contributed to her death in the context of diving at depths in and around the wreck of SS President Coolidge," the coroner said.
"There were noted faults in the dive cylinder, which was in poor repair."
When Dr Osunsade was found, after becoming separated from her guide, her fins, subsequently found to be too big, were missing.
Coroner Shortland also noted inconsistencies in the guide's version of events.
Dr Osunsade was carrying too much counter-weight and had, on dives the week before, been seen breathing rapidly and had cut a dive short because she was low on air.
"On the balance of probabilities, Dr Osunsade was most likely under stress and anxious during her fatal dive. She had ill-fitting equipment, namely extra large boots and fins, which resulted in them coming off her feet," Mr Shortland said.
"Clearly she had been over-weighted and had to work harder in the water to maintain appropriate buoyancy. The ill-fitting boots and fins would have added to the fatigue, alarm and stress she would have been experiencing."
All that and her being left alone led to her death, the coroner ruled.
At the time of her death, her husband, Damien Healy, said he was devastated.
"I've lost the most beautiful person in the world."
Dr Osunsade, an American, moved to New Zealand four years before the tragedy. She was an ear, nose and throat specialist.
Her body was returned to New Zealand in May 2013.