Dan Thomas said he and Mr Petrenko had agreed to return to their boat after 30 minutes but Mr Petrenko didn't arrive.
In a Facebook post, Mr Thomas said: "We spotted his light in the water and after a min[ute] of watching it with no movement I dived in."
But he couldn't reach his friend and after a few minutes the diving party saw Mr Petrenko's torch drop to the sea-floor.
Thinking it might be dangerous, the rest of the group left the water.
"Police divers said that the rock which Igor was diving in had a suction current and pulled both divers under," Mr Thomas said.
"My thoughts, my love and my whole-hearted apology goes to the loved ones of both."
A police spokeswoman said the alarm was raised after a diver did not resurface, and a second diver who went to investigate also disappeared.