This might belong more in the Beginner's forum, and I'm not much past a beginner myself, but I can try to answer from my own first-DSLR experience (rig IDed in signature below). I stepped up to my present setup from a Canon A520 in the Canon housing, which likely had similar performance to the Sealife.
I can't speak for the Canon Rebel specifically but in my Oly E330, which is a pretty 'entry level' DSLR a couple years old, I have only 3 focal spots and I more often than not turn off the outside ones and force it to focus on the center only. If I want to frame a shot with the main element off-center, I do the half-press-to-lock focus and then reframe before completing the shot. You would perhaps find yourself using that technique as well. I think even for a newer DSLR with a bazillion focus points, if you're after a wide aperture shot and narrow DOF, you have to be careful about what the 'autofocus' ends up grabbing. But of course with DSLR the lens and sensor are both bigger, so you can get away with a smaller aperture and wider DOF and still get the exposure, so for those shots more focal points definitely equates to a faster overall shot. (Plus if you go up to a DSLR I'm assuming you'd also add at least one external strobe.) The other thing that can help focus speed is a 'modeling' or 'focus' light - doesn't have to be one of the high-end, $400 ones, just basically a flashlight that helps the AF lock better. It's not much intensity relative to a strobe so you don't really have to worry about the fancy autoshutoff and such features until you get to the higher end...
In terms of viewfinders, most people say that Ike's can be a bit challenging, but are usable. External VFs can be added to ike housings, but at considerable cost. It will take some adjustment to use it vs. the back screen of the Sealife. Disadvantages is of course you're seeing the world thru the viewfinder while pursuing the shot, but what you make up in faster AF and immediate shutter response relative to the PnS should significantly assist gettting shots. I would not from my reading recommend using liveview modes on Canon right now, most of them are really more intended for 'tripod' type work. If you really want Liveview then your best current options (pending housing availability) are the newer Sony DSLRs* (A350 and up) or the micro 4:3rds Panasonics**...they're the only ones right now that are doing a much higher speed live view that doesn't sacrifice a lot of the advantages of DSLR in terms of either much slower focus or additional mirror actuations which equates to shutter release delay. (Confession - I'm still almost entirely a liveview shooter myself.)
I can tell you that using my own first DSLR has been both a 'joy' compared to the old point n shoot - and a horror as I STILL miss shots, and the ones that do come out are so much closer to what I want to be taking that it's teasing me to ramp up the hardware curve further to get the others I just "know" I "should have". But I know as a vacation-only shooter, I have to get my skills up further before I really upgrade much more aside from perhaps larger strobes. (Course I can always upgrade to a better body for topside practicing... 
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My recommendation, if you know your Rebel and are happy with the topside pics it takes now, practice trying pics with relatively wider apertures (f5-f7) in medium to dimmer light and get a feel for how well it will focus, meter, etc. for you. Maybe trying to shoot things like a single leaf partially obscured behind another tree limb or whatnot, kind of like trying to focus on a given eel (or banded shrimp) hidden behind gorgonian limbs (or in a barrel)...if you can find a shooting method that works for you under those sorts of conditions, that's analogous to what you'd be experiencing when shooting underwater...so go look for a housing.
There's always going to be people who say "buy used first and work your way up as your skill increases instead of breaking the bank at the beginning", and those that say "why pay incrementally and ultimately more vs. just buying it 'right' the first time?" I'm strictly in the bottom feeder category myself, at my skill level, plus cameras change way to fast to ever 'keep up' with the 'best' choice at any given time, but you've gotta make that call on your own. If you like your current camera, you can probably house it and get some used strobes for a sum total of $1500 or so. A newer body and housing alone will greatly exceed that, even if you get frugal with used strobes and accessories on top of it. ebay and the classified sections here are great places to score used Ike housings.
Welcome to the madness.
Footnotes:
*The Sony live-view is using a secondary sensor up in the viewfinder area, with an optional mirror-tilt position that feeds it vs. feeding the optical viewfinder. So 'live view' does not require extra actuations moving the primary mirror out of the way of the main phase-detect AF or for metering etc...the camera essentially starts from the same point when taking a pic using the optical viewfinder vs. using the liveview. The older Oly E330 that I have did it in similar fashion with a partially silvered mirror feeding both optical viewfinder and secondary sensor, at the cost of some viewfinder brightness and size, and the subsequent Olys (420, 520, 620, E30, etc) have all abandoned the 2nd sensor completely and are using various generations of contrast-detect AF instead of or in supplement to phase detect AF to try and minimize the shutter delay consequence of their liveview modes. Canon and Nikon both to my knowledge use liveview modes that also require more mirror actuation for metering and/or focusing, although the new video-capable ones might let you essentially 'frame capture' from video, but I don't know at what rez or how that would work with lighting, focusing, etc. With liveview remember you may also be gaining a 'video lag' in terms of waht you see on the electronic viewfinder vs. the actual state of the subject, in addition to any extra actuation delay necessitated by additonal mirror flips, slower contrast vs. phase detect AF, etc. (That's something I didn't appreciate at first and used to argue against, but having used it for a while now I can definitely say it's true.) Bottom line, Right now DSLR "liveview" is a buzzword that not all manufacturers are using in remotely the same way to describe equal capabilities, so read up on the actual implementation tradeoffs for any proposed camera choice!!
**Panasonic micro4:3rds G1 and GH1 - the "DSLR-like" form factors, reportedly have very fast electronic viewfinding and fast focus. Housing is available for the GH1 but is pricey (non-Ike). Their smaller GF1 also has reportedly very good focus performance, but no housing yet I believe. The current Olympus E-P1 reportedly has fairly slower focus performance, but Oly is rumored to make an E-P2 announcement on 31 October. There is a company right now engineering a housing for the E-P1 that will use Nikonos lenses, which is an interesting combination, but that will require manual focus work with the lens.
