"And that's when I turned back and could no longer see Bethany."
McTavish said she took Melissa Clark, Farrell's companion, to the surface and then spent an hour searching before spotting Farrell "beyond the diving track in deeper water".
Water conditions at the time were later described by local divers as "terrible", with a visibility of about 1.8m.
A search operation was launched and 30 minutes later Farrell's body was found on the seafloor at a depth of 15m.
But it has since emerged that Farrell made it to the surface before drowning.
Phillip Brown, a crew member of the other tour group, said he saw Farrell at the surface, waving in distress.
"They were grappling the water, trying to grasp for air," he said.
Farrell's dive computer apparently showed that she may have been at the surface for up to 40 seconds before losing buoyancy.
Patrick and Caron Farrell, her parents, said they believed the death was "completely avoidable" and resulted from a "string of systemic failings".
"No stone will be left unturned in our fight for truth and justice for Bethany,'' Farrell told the Courier Mail.
Patrick Farrell said he was hoping the inquest would lead to an overhaul in work practices and legal action against anyone found at fault.
The coroner is expected to examine training for novice divers, safety standards for instructors and procedures for dives in poor conditions.
An initial investigation served the tour group with a non-recorded fine of £90,000 for "general supervisory failings".
The inquest is due to continue tomorrow.
