OF--First off, welcome to SB! I'm not sure if anyone but Ikelite is making a housing for this camera. (Figure to pay around $680 for one from places like B&H Photo.) That said, an external strobe would almost be mandatory and if you go w/ an Ike housing you'll be using their strobes. Not that that is a bad thing, just an "expensive thing".
Some folks can get by ok w/out an external strobe but that is usually because they have use of their on-board camera flash and then only if they are in close. The Ike housing won't allow your flash to pop up and fire, so...On the bright side (literally), for about a 100 bucks less than the Ike housing alone you can get a Canon S95 (around $400.) that has great low light capacity and a Canon housing (around $180.). I'm just sayin'...
Check the other threads here for input on the S95--it's a really hot UW camera to hear tell. // ww