Well, that depends really. I also shoot 550D topside, but I don't take it underwater (still using my old G9 because it is just fine, I don't want to spend the money to upgrade).
Some questions:
- What's your budget? Shooting DSLR UW can be a steep starting point (and I don't want to buy your 2nd hand housing if you find out you hate it after 2 months, well maybe...)
- Lenses is all about (budget and) what you want to shoot: wide angle versus macro is a huge difference. Which ones do you have already? Where are you going and what kind of marine life are you expecting to see or likely to photograph? With lenses come domes, be prepared for that too...
- Besides housing and lenses, have you thought about strobe light(s) too?
Just as a personal opinion (feel free to disagree), I'm not sure a 7 year old (not top-of-the-line) DSLR is the right "starting point" for an UW photograpy adventure. Topside it wouldn't really matter, but the UW housing adds a lot to the price, and personally I wouldn't be ready to spend that money. Also, I'd say there is a lot on the market today that will outperform your 550D, the first things that come to my mind are focus and video (if you care about that).
If you can find a housing for a reasonable price (and i think all the brands you mention are fine, I like Nauticam but the others are fine as well), I think that can be a good start. I'd add a strobe to that first, and even with the kitlense (18-55) - if it fits the housing - you will be able to take very nice pictures. From there, there is plenty to invest 