I've cruised several times, and would do it again. I even got a norovirus once, but recovered within 12 hours and had a great time.
Fire on a ship is one of the worst things that can happen. Even on a metal ship, you have oily rags, fuel under high pressure, hot exhaust, laundry rooms with lint and hot dryers, rope lockers, etc.
The newer ships are typically built with two independent engine rooms. Still, there's usually a common electrical switching panel, and if there's a fire in that compartment, most everything is off line.
There are smaller generators to power emergency lighting and bridge navigation and communication systems. Remember the pictures of the Concordia with all the deck and nav lights on?
Sounds like they even got an elevator, the potable water system and some of the heads working on Triumph.
It seems odd that they don't have sufficient backup generator capacity to run the basic life-support systems like the waste water system. Even if they only backed up a few heads in each area of the ship, they could avoid the inevitable "fecal matter" references that we read in the paper every time there's a crippled ship.