erm actually before buying the casing and all. u must ask yourself how often do u dive...
if u're gonna dive once or twice one year... i don't think its advicable to buy such expensive cameras/set up.
u might get a WOw quality picture with a dslr but a compact camera is good enough to help u remember the wOw moment...
maybe u can think about those 950 Is series with canon casing. should be around 700+ in total then invest the rest of the $300 on a huge underwater torch.. just need someone to hold it for u when u're taking a picture