Three men and a woman died when their AS332-L2 Super Puma aircraft crashed into the water on August 23.
It was on its approach to Sumburgh Airport with 16 passengers and two crew when it ditched into the sea around 1.5 nautical miles from the runway after losing speed on approach.
Death certificates show that Sarah Darnley, 45, from Elgin, Duncan Munro, 46, from Bishop Auckland, and George Allison, 57, from Winchester, all drowned. Gary McCrossan, 59, from Inverness, suffered heart failure.
RMT union regional organiser Jake Molloy said: "We don't know what the circumstances were.
"Were they conscious, were they unconscious, were they debilitated or had they been struck? Did their survival suits or life-jackets work?
"It would be inappropriate to comment further until we get the Air Accidents Investigation Branch (AAIB) report."
The Eurocopter aircraft had collected Total passengers from the operator's Dunbar platform, the Borgsten Dolphin rig and the Alwyn North platform before heading to refuel at Sumburgh.
Most of the passengers on board managed to scramble to safety along with the two crew and were picked up by emergency services sent to the scene.
The latest interim AAIB report described the impact as "survivable".
So far the investigation has also failed to find any trace of a technical fault.
It was the fifth helicopter incident to have happened in the last four years.
The same model of the Super Puma was involved in a ditching in 2009 when all 16 died.
The full AAIB report is due early next year.