something on the central console
Yep the transponder.
The APU generator supplies 120 KVA electrical power at any altitude.
The left main fuel tank must contain a minimum of 950 pounds (430 kg) of fuel to start and operate the APU for one hour. The APU uses 675 lb (306 kg) of fuel for each hour of APU operation after the first hour.
So your 1000 kgs of fuel isnt going to last long.
The RAT generator supplies 7.5 kva of ac power to the two center TRUs. The RAT GCU monitors and controls the output of the RAT. The TRUs change the ac power into 28v dc power
It rotates at something like 4510 rpm and the blades are 105CM long.
So the RAT would be your best choice of power.
As for ditching close to a beach, i don't think that anyone would try this with the gear down!