I have a 5D but have never been tempted to take it underwater, for several reasons:-
It's a heck of a lot of money to lose when it floods. Be sure that at some point it will - I know lots of u/w photographers, some professional, and they have all lost at least one setup to flooding. I myself have lost two. A friend (professional) lost an almost new HDV video camera last year, and you could not meet anyone more fastidious over gear preparation than he was.
I haven't come near to exhausting the potential of a decent compact yet. I think it's far more rewarding to get really good pictures from fairly basic equipment than just to throw an awful lot of money at it and probably end up with mediocre results. As a resort dive operator, I see lots of people taking many thousands of $ down with them, and there is absolutely no correlation between what they spent and what they achieve.
In any case, if you're primarily intending this camera for underwater use are you sure the 5D is the best camera? It's a wonderful camera capable of superb results, but it's much more suited to studio or fairly static use than to sports. It's also quite bulky and heavy, and as it's full frame the lenses it takes are also bulky and heavy. And expensive - you'll have a far wider choice of affordable lenses if you go for a cropped sensor. Underwater I doubt if you can get results justifying the full frame sensor.
Also remember that the 5D has been around three years and technology has moved on a bit. In particular it doesn't have live view, which I think you may find useful underwater. I don't want to put you off it or Canon, but have a look at the Nikon D300 and D700 before you choose.
If I were you I'd upgrade to something like a G9, which with the Canon housing is quite affordable and with the Ikelite housing (which goes deeper) isn't too bad. It takes RAW (does the G5?), which underwater you'll need. You'll also need decent software and the knowledge and experience to use it.
Don't forget that to justify a DSLR underwater you'll also need double strobes, and those alone will set you back a lot of money.
Why exactly do you need a DSLR for your photography classes? Will these classes be of static object or of fast moving sporting events? And how strong are you? I use my 5D with (inter alia) Canon's 100-400L lens, and that package isn't for wimps. A friend with a Nikon D80 gets 50% more reach with a camera/lens package that weighs about 1/3 of mine and cost a fraction. And he gets superb results.
