I tried the same thing with my C5050 and a Ikelite Auto35 Strobe, never got a chance to try it underwater because it flooded on the first dive. It fired fine and did a pretty good job on land. You do have to turn off the preflash on your camera or it will not work. On land I had problems with overexposure, especially at close ranges. Based on my land experiments, you may need to build some diffusers to get the strobe power in the correct range for some shots. You may also want to build some kind of strobe block for you Cannon housing to block the internal strobe and to connect the fiber cable to. This will reduce back scatter for wider angle shots. I made one for my C5050 that is removable so I can also use the internal strobe on macro shots.
Ultimately you will want a better strobe but why not give it a try.