The Times of Malta reports that McGahern - who was born in England but raised in Ireland before moving to Malta 17 years ago - entered the water at the Starfish Diving School before midnight last night, hoping to break the standing record of 11 hours and 46 minutes.
His previous attempt at the record was ended little more than an hour short of the record due to bad weather, but today's clear forecast has buoyed his confidence.
McGahern plans to pass the time by cleaning the seabed, assisted by a team of 16 safety divers, but he also intended to catch some sleep below the depths.
“I’ve slept underwater before," he said. "it’s not as difficult as you might think.”
McGahern, who previously held the warm water open sea dive record, is undertaking the challenge for Dar tal-Providenza, a home for the disabled on the Mediterranean island.