Lonewolf, I’m assuming if you were starting the APU for winching ops for utility hydraulics, the hoist motors were powered by hydraulics? Us Army H-60 hoists are all electric now, though I think in the past some were hydraulic and Navy systems may still be?
Remember also that the battery utility bus powers the fire suppression system for #1 Eng and APU, converted AC power to the DC primary bus is required for the #2 Eng fire suppression to work.
I used to question this lack of redundancy but it’s probably the best compromise in systems design to handle the most likely scenarios for dealing with fires, meaning a situation that ends up with your #2 engine on fire with no converted AC power to fight it means a lot more has gone wrong in your day than just a #2 engine fire.
I think you pointed out, a situation that droops the rotors below 90% that kicks off both generators or cascading failure of the AC electrical system (both AC generators and the APU generators all failing together), would seem unlikely and a power on ditching is probably what happened in this situation, so it becomes more of a possibility of a human factors or SOP issue why the floats weren’t deployed than a systems failure.
FltMech