A yacht of that size should have had redundancies built in AND a manual override.....
Yes, you are right on. My friend has a 55 foot Nordhavn and even my own boat There is only one hydraulic rod/piston and it is ether controlled by the GPS, autopilot or manually. The GPS system and A.P. will control the hydraulic pump. Manual, you are turning the wheel which acts as a pump to move the rod which moves the rudder.
Thats not to say, that this is the same setup in a 250 footer. But generally, thats how it works in steering.
If it was the engines, then why not shut them down. But is we all know, no brakes. Generally, its a fly by wire system. Was it one of the computers on each engine or is there a master computer to control all of them?