Dr Colyn and bkkchris,
I think as bkkchris you'd be hard pressed to tell any photographic difference in images taken with any of these camera. I've shot all (OK, T2i as I haven't tried the T3i yet, but same sensor is in all) and as bkkchris says the lens choices is key.
I currently own a 60D mainly because I wear glasses and for surface use I found the PentaPRISM viewfinder and all 9 AF focus points being cross type to be a tad better than the T21i / T3i (550D / 600D) model. Not a huge difference as I've used every generation Rebel series, plus 20D, 40D, even 5D above and underwater in Ikelite housings.
Ikelite housing are an incredible value and let me address the issues many post about in real world use plus Canon model differences.....
* Canon 7D is weatherproof and has a 100% viewfinder in eyepiece Very, very nice.....The 7D shoot 8FPS (frames per second) but this is more for surface sports shooting than UW. That said I used a 7D for a competitive swimmer shoot (see Ikelite BLOG post at
IKELITE Underwater Systems) Great AF with almost too much menu choices to set up. Many buy it and struggle until reading the manual many times. This camera is also HEAVY and yes, claims 100% weatherproof (see comments below about this.)
* Canon 60D has a 96% PentaPRISM Viewfinder, plenty fast AF speed and will do 5.3 FPS. Lighter, has front and rear command dials and the main feature I chose it for last fall. Wireless surface flash in eTTL with Canon flash units off camera

FYI the NEW Canon 600D / T3i has added this feature now......Lighter weight than a 7D and feels a bit more substantial than T2i / T3i and at least $500.00 less than a 7D.
* Canon 600D / T3i has all features less multiple configuration AF and no rear command dial. This is not as big a deal as people think......Ikelite housings and Nauticam and others have a control that is held in to depress the rear button making the front command dial the f-stop adjustment.
You'll change f-stop more after selecting shutter speed for wide angle background brightness or for macro a dark background. Lightest camera of the bunch, responsive enough, 96% PentaMIRROR viewfinder (not as bright as 60D or 7D but certainly usable) and a joy to carry all day.
I've used many lower price bodies (you can buy TWO for back up and trips) and the always touted "build quality and weatherproofness" is ridiculous in my opinion. If you totally flood your housing in salt water your camera will be toast anyway, even a 7D and / or the lens on it. I guarantee it......
I don't about you but I stand in a pouring rain storm taking photos unless I'm under an umbrella or have weatherproof "snoot" type gizmo covering my whole camera except the lens sticking out. Hell, a lady's shower cap in hotel bathrooms has served as rain bonnet a more than once
Lesnes are key and the most recommended to start are Tokina 10-17mm Fisheye or Sigma 15mm F2.8 Fisheye for wide angle. Then Canon 60mm for macro. The Canon 100mm Macro allows a bit more working room but will challenge new SLR shooters underwater viewing and framing tiny subjects.
Finally, housings..........
Yes, I'm an Ikelite dealer but also avid USER of their housings. I think they are a great value allowing you to SEE your camera is safe inside. Many folks use a camera body for a few years and then only upgrade to the NEWEST selling the camera body and housing body. All ports, cords, strobes and arms will work TTL flash wise with future models.
I've seen and used many other brands. Many are works of art but also do not offer TTL / Manual flash control from housing circuitry just like you have a dedicated flash in your hot shoe. You can over ride to manual if you like to also.
Fiber optic tripping of flashes (Optical synch as it's called) is popular with some brands but you will have to fire your built in flash each time. Many dial this down to simply trip their external units and get good results once they know what MANAUL settings they need for f-stop and distances. I think TTL is much, much better and shoot it on WA and macro. those who pooh pooh TTL on WA have never shot a system UW with it.....
Many posters will also say Fiber Optic cords are cheaper and more reliable. I disagree......too each his own.
Some of the other brand housings controls are closer, sexier, etc. but in real world use anyone who has used and practiced with their camera will be adjusting settings a lot less in the brief time we have UW. Many lead people to believe they need to adjust multiple settings the whole dive to get good images.
IMHO this is just my real world experience helping many, many Ikelite users get great photos UW with any model camera.
So I guess my recommendation if you want front / rear command dials, a slightly better viewfinder is go with the 60D. Right now the darn thing is within like $30.00 USD of the newest announced 600D / T3i.
If you'll shoot sports above water (8FPS) and want the 100% viewfinder plus can deal with weight for travel (a HUGE pet peeve of mine is always reducing weight wherever I can) then go for the 7D.
If the 600D / T3i comes down a bit soon buy 2
Hope this didn't confuse you and gave another perspective.
David Haas
David Haas Underwater Photography