The Air Accidents Investigation Branch (AAIB) has found a lifeboat could not leave a slipway west of Sumburgh Airport because of tidal conditions after the Super Puma came down nearby in August this year.
The AAIB's report, published yesterday, found that Coastguard rescuers were quickly on the scene.
But it found a Fast Rescue Craft arrived 58 minutes later after it was forced to switch slipways, to one south of the airport.
It had been unable to enter the water at a slipway to the west of the area.
The helicopter was ferrying workers back from the North Sea when it suddenly plunged into the water, killing Duncan Munro, 46, George Allison, 57, Sarah Darnley, 45, and Gary McCrossan, 59. Fourteen people were taken to hospital.
The report revealed three of the fast rescue craft's crew members suffered injuries in the difficult conditions.
The AAIB is recommending the Civil Aviation Authority "reviews the risks associated with the current water rescue provision for the area of sea to the west of Sumburgh Airport and takes appropriate action".
The AAIB found no evidence of a technical failure in the crashed helicopter, adding that the engines were still running when it hit the water.
The flight approach of the Super Puma is said to have been normal until three miles from the airport.
All passengers in the helicopter were wearing survival suits but the crew were not.
It recommends that Sumburgh's operator, Highlands & Islands Airports Limited, provide a water rescue system suitable for all tidal conditions.
The launch slipway to the west of the airport is "shorter and narrower than optimum", meaning it can only be used in 11% of tidal conditions.
The report said: "This accident has highlighted that, in the majority of tidal conditions, the FRC may not be able to respond to aircraft accidents in the sea on the western side of Sumburgh Airport within the available survival time.
"It is recommended that the operator of Sumburgh Airport, Highlands & Islands Airports Limited, provides a water rescue capability, suitable for all tidal conditions, for the area of sea to the west of Sumburgh, appropriate to the hazard and risk, for times when the weather conditions and sea state are conducive to such rescue operations."
Sumburgh Airport is required to have appropriate resources for open-water rescues within 0.6 miles of the end of its runways.
The helicopter crash happened beyond the airport's response area but crews launched the FRC to help the coastguard.
A Civil Aviation Authority spokesman has already announced a review of offshore helicopters. It said the authority was continuing to support the investigations.
A spokesman added: "Our review of offshore helicopter operations, which we announced on September 24, will thoroughly examine the risks and hazards of operating in the North Sea and how these can be managed most effectively."