If this was a GFS operational flight then I doubt that operating at OEI weights was much of a consideration. Training yes - operations no. Even with only 3 crew onboard if it was full of fuel and hot and high in a low hover picking up a heavy load of water then ditching would have been the only option for the Super Puma.
Same for SAR. This is not CAT A passenger carrying flying. You fill up to get the job done - not to plan for an OEI fly away.
The crew would have briefed the OEI scenario after observing the hover TQ at the start of the lift and would have ben expecting to ditch in the event of OEI. Which seems to be exactly what they did.
surely the aircraft could have flown away on one engine
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Short shaft - you would probably be asking why they didn't ditch if they had attempted to fly away on one engine and crashed.