Rescue bid killed 2 sailors.
They tried to rescue Polish crewman Robert Ebertowski who had collapsed in the confined space of the anchor chain locker.
Coxswains Finlay MacFadyen, 46, of Aberdeen, and Robert O'Brien, 59, of Leven in Fife, followed him to their doom.
Mr Ebertowski and an unnamed colleague had been sent to stop the anchor chain rattling aboard the Viking Islay rig support boat.
The report says: "One of the seamen entered the chain locker and collapsed.
"It is probable that the other seaman, realising that help was urgently required, raised the alarm with the duty rating on the bridge before he, too, entered the locker in a bid to help his companion. He also collapsed.
"During the consequent rescue efforts, the first rescuer found he was unable to enter the chain locker wearing a breathing apparatus and he, donned an emergency escape breathing device.
"He entered the space but at some point the hood of the EEBD was removed, or became dislodged and this rating also collapsed."
Today's Marine Accident Investigation Board report blamed boat owners Vroon Offshore Services for not picking up on deficiencies in "training, equipment and safety culture" on the vessel.
It said the boat had unsuitable gas monitoring systems.
The accident happened in the North Sea 25 miles off Yorkshire last September.
VESSEL: Viking Islay