first decide on a price point, ikelite on the low end (maybe 2.5-3g's for a complete setup), subal & seacam on the high end (maybe 8-10g's+) - look at size, weight, port systems, lens variety supported, availability of a large viewfinder (absolute necess IMHO), and location for when you need service.
Ikelite's cheap and has great customer service, but bulky and port & arm systems aren't great, aquatica's decently priced w/ less bulk, but has occasional QC issues, subal's a big favorite, quality, lots of accesories, small & lightweight but expensive, seatool's amazingly small & light but accomodates less lenses. And others like Sea&Sea, Nexus, Hugyfot, etc. I don't know much about.
There's a lot of great information in old threads, do some searching through the forums.
Personally I went for Aquatica for these reasons - lower price point, but still a form fitting aluminum housing (less bulky), a good port/extension ring system, and they're the located closest to me. Now they have their big viewfinder (expensive but oh so worth it) and a new 9.25" glass dome (mine is en-route from b&h), adding ability only before found in the high-end makers.
Realize that once you've chosen, you're likely stuck with that brand for future upgrades once you have all the bits & pieces. All these bits & pieces get bloody expensive - plan on $1500+ for strobe arms, focus/zoom rings, port extensions, spare orings, sync cables etc.
The other thing is lenses. Your kit lens is usable but not ideal UW. Primary lenses of choice are fisheyes(FE) and macros. For a canon, the best starting package would be either the tokina 10-17mm FE or the canon/sigma 15mm FE, and the canon 60mm macro. Some use rectilinear ultrawides (canon 10-22mm) but struggle with close focus ability and soft corners.
Hope that helps for a start. Cheers,
Chris