While I do take Alex's point, and in my heart wish I could agree with him, I'm firmly in the have a back-up school. I've had cameras fail on a trip. I've seen other people have their sole camera fail on the 2ns or 3rd day of diving on a trip. It sure was nice having someone with such good eyes acting as a spotter for the rest of the trip.
Having a serious body always available for topside shots is important as well. (When a whale suddenly broaches or hoves alongside is not when I want to be scrambling to get my camera out of its housing, switch lens, etc.) So the effective cost of that back-up body, to me, is the difference between the the cost of the 2nd D2X and, say, a D200. Still nothing to sneeze at.
When I had a film housing, I had two F801s bodies, and I had a second Fuji S2's for my first digital housing as well.
I also carry two complete sets of strobes. I normally use three strobes for wide-angle, two for macro, so basically I'm carrying one extra strobe for redundancy. At least one spare set of synch cords.
While on the subject of redundancy, I dive with two computers (Diverite Nitek Duo and a Nitek Pro, which are identical to the Apeks Quantum and Pulse computers). Unless I know the operator has good rental/lend gear on board, I also carry a spare regulator set, or at a minimum, a spare SPG and back-ups for every hose.