Water temperature is not the only element that needs to be considered where survival is concerned - adequate clothing (whether or not survival suits are worn), survival suits and splash-hoods on the lifevest (or on the survival suit if it is integrated) also improve the chances of survival.
The 407 accident that led to the death of two of the occupants in February 2003 occured in seas of 5ft - 9ft and with winds of 25 - 40 mph. Existing guidance indicates that the survival time can be reduced by two thirds with winds above 25 mph. This would put the survival time at less than 30 minutes in sea temperatures of 13C.
Sea states with breaking waves can also reduce the survival time unless spray hoods are deployed (spray hoods, on lifevests or suvival suits, are mandatary for offshore flying in Europe).
You already know that there have been deaths due to exposure in the GOM. If the causal chain contains night flying, high seas, strong winds and single engine operations; one link that is preventing a hazardous event is the reliability of the engine (or any other cause of ditching). With the assumed engine reliability of 1:100,000 (if in fact that is achieved) and usage rates of 400,000 flying hours per year in the GOM, there is the potential for four fatal accidents a year due to engine failure (in addition to those which are not engine related).
The causal chain can be broken in a number of ways but the elimination of single engine night flying and the introduction of an adverse weather policy would be two which could have most effect.