I know I've tried to find this in the past but it seems like some 10 years ago nobody wanted to rent me any housings because of liability issues. They would only rent a camera + housing, not just a housing for my own camera.
I stopped caring for a long time using a small Konica Mermaid film camera for a while when I was on holiday and after that I used my CP995 in an underwater protaction bag. Since I got my Fuji F31fd I bought myself a dedicated underwater housing and can use my camera till as deep as I'm allowed to go (though I do mainly snorkling). For me right now there is no real need to spend a lot of money on an underwater housing for my dSLRs, the results for snorkling are just amazing with the small Fuji as long as you stick to tropical waters. Otherwise an external flash would come in handy, even those you can buy relatively cheap and can be (re)used on all sorts of future or different setups.
My advise, try with a compact and underwater housing first and if you are really into it doing a dive per week, then consider if it's worth the money to buy one for your dSLR. Don't count on renting one where you'll be diving though.