I've personally looked into this before and am friends with a professional cave diving photographer who uses an Aquatica housing for her 5D MkII.
There are lots of available options for underwater housings for DSLR's. Unfortunately, as previously mentioned, they cost almost as much as the bodies themselves. In addition, you need to find a port that matches you lens. Housings for the 5D Mk II cost between $1,700 and $5,000 There are other mid range options available from a number of manufacturers.
Since you didn't specify depth i think it's worthwhile mentioning 'camera dry bags' which are usually good to a depth of 10-20 meters depending on the make and model. Some of these bags have room for a flash to sit inside it as well however this might not be ideal for underwater photography. These bags usually cost around the $100 mark.
Finally, with regards to strobes, there are multitudes of options, but you need to buy the right strobe for the right housing. Ikelite strobes work with ikelite housings etc.
A couple of example strobes for Ikelite and seacam
Prices vary between a few hundred and $2500+ for a strobe.
I would recommend hiring out any gear you plan to buy for a few trips before you commit. It is, like all good camera things, a significant investment.
Additionally, something to be aware of, if the lens ports get scratched you'll need to get them professionally buffed out which can cost several hundred dollars (almost as much as a new port in some cases).
I hope that helps. If you have any further questions feel free to ask!