Fellow diver Shannon told 9news.com.au he was aboard a Rottnest Fast Ferry at around 12pm (3pm AEDT) on March 2 when he saw two men in the ocean waving in distress. 

An investigation is now under way after the pair were allegedly left behind while on a dive organised by Perth Diving Academy (PDA), based at Hillarys Boat Harbour, while around two nautical miles off-shore.

“Coming back from Rotto [Rottnest Island] about midday noticed two black things bobbing on the water,” Shannon explained.

“As we got closer I could see it was divers with no boat around or even close to where they were. 

“I instantly tapped on the captain’s window and pointed them out, [he] stopped the ferry and notified everyone that we were turning around to get them.”

Shannon – who has 15 years of diving experience himself – said the pair were lucky they weren’t struck by a passing ferry as they waited to be rescued.

“They had no surface marker buoy that was visible, which should be the bare minimum for attracting attention on the surface and for the safety of visibility to prevent being hit by an oncoming boat,” he said.

“What surprised me was one had a dive slate and usually only instructors have that if they are teaching a course so they can write on it and communicate underwater.”

He added: “I’d be hugely concerned if that person was an instructor for PDA.”

Shannon said one of the divers jumped onto the ferry, while the other was taken back by the tour operator’s boat.

Perth Diving Academy’s Wildcat had been radioed by the Rottnest Fast Ferries crew shortly after the men were found.

“They were nowhere in sight until about 10 minutes after the divers were spotted,” he added.

The Australian Maritime Safety Authority (AMSA) said it was aware of the incident and was investigating.

“Incidents involving passengers on board Domestic Commercial Vessels (DCVs) must be reported to AMSA,” a spokesperson told 9news.com.au.

“AMSA takes the safety of passengers and crew very seriously and will not hesitate to take swift and strong action should a breach of safety regulations or procedures be identified.”

One of the men needed hospital treatment following the incident.

“When that guy got on the ferry he looked exhausted,” Shannon added.

AMSA officers yesterday visited the diving company to inspect the boat and talk to its director, Troy Lane.

Perth Diving Academy said it was unable to comment as AMSA was investigating the incident.

Marine safety laws require regulated commercial vessels to file an incident alert within four hours and provide further detail in a report within three days.

AMSA hasn’t revealed when or who brought this rescue to their attention but it says swift and strong action will be taken if any breaches are identified.