For a belt and braces approach I've added a flash connection to my housing which operates through the PC socket on the camera. I am admittedly, fortunate enough to have a spare blanked off socket, and have added a Nikonos socket here. I have wired this internally (soldered - essential) to half of a Nikon SC11 cord which plugs into, and then screws tight into, the PC socket on my camera. As the collar of the PC socket (Canon camera) uses a threaded collar which the SC11 screws into, the connection cannot come undone underwater. The connection merely require that the circuit is made, and whilst only straightforward synchronisation (not rear curtain) can be used, it works fine and acts as a back-up should any problem develop with the main circuit.
In my experience, flash circuits are the most prone area of a housing to fail, so an isolated spare is well worth having.
