Getting a housing a great first step!
But neither of your lenses are really suitable for underwater work, so I would also invest in some glass and the correct port.
As you will be shooting without strobes, I'd be tempted to say head to WA more so than macro. Maybe the Sigma 17-70 which will be a good lens even after you get the strobe. Or the Canon 10-22/Sigma 10-20 - both are very good choices, too.
The magic filter idea is a good one IF you will be shooting in the conditions it is designed for. Check out their site for more details.
Personally I can't imagine using my camera without a strobe, but that's dictated a lot by the type of shooting I really enjoy. I'd be buying a strobe ASAP or I think you're going to find your self quite frustrated with the limitations of natural light...depending on where you shoot and what you like to shoot, of course!
Check out the pink link in my signature as there is a short list of lenses and details about why they work.