If you read the CAA report, you will see that the high level floatation systems were in addition to the standard floatation bags. Various options were considered, including high buoyancy foam incorporated into the cowlings etc. These would help to prevent a total capsize, holding the fuselage in a partially capsized position, creating a large air pocket inside the cabin. Admittedly, one side of the cabin would be low and one side high, but trials carried out using the system proved successful.
The floats were only one of the recommendations in the report: there were several others.
I suggest anyone interested have a look at the report.
CAA Paper 2005/06: Summary Report on Helicopter Ditching and Crashworthiness Research | Publications | About the CAA
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