Go for the digital!! No matter what you are going to do with the photos.
The external strobe on the MX10 is fixed so all the light goes straight out and straight back into the lens so backscatter is a problem - there's no controlling the angel, and I don't think you can control output levels either. It is a good beginner's rig, but the learning curve will be exponentially higher than if you simply go digital.
There is more in it right away in digital in this case, but you have no processing costs and no film cost - so what, at least $10/roll total? That's 15 rolls of film difference...you probably won't be getting as good of results after 15 rolls as you would from two or three dives with the digital even without the external strobe. The bigger cost is what are your dives worth? If you shoot a whole roll that doesn't work out how much is that worth to you. With digital you see your results right away and can make adjustments on the spot.
The best benefit, IMHO, to digital right away is the learning curve. You set up, shoot, review in the lcd panel, adjust your settings, shoot, review, adjust, shoot, review...this way you can get the shot right on the same dive instead of having to wait to get the roll back only to find that you were doing the same thing wrong over and over!
The internal flash on the canons is very good - you must always work within the limits of the camera. Have a look in my gallery and lukerob's gallery for some images from various Canon A series cameras...I know there are several others here, too, but I can't remember right off the top of my head.
HTH