I actually bought a housing for my backup camera (40D). After messing around with it, I realized that I was better off housing my primary (5D2). Ended up exchanging it for the 5D2 housing. Of course, that meant a little more money. However, the FF capabilities really worked for me. I have a 15mm Fisheye for UWA and CFWA shots and a 100mm macro for critters. Also, withe the Ikelite 8" Dome, I was able to put on teleconverter for the Fish for tighter shots. I'm sure the 7D is a MUCH better camera than the 40D, but here's my rationale:
Housing 40D Pros:
Cheaper
Can use Tokina 10-17 Fisheye zoom (flexible focal length)
Tighter macro shots (due to 1.6 crop factor)
Excellent low light focusing
Cons:
No video
Subpar LCD display
Lower Mega Pix count means less cropping
Unfamiliar handling since I am used to the 5D2
Housing 5D2 Pros:
21 Megapixel = blowing up my pictures BIG with good pixel density
Cropping capabilities
Video
Used to the camera since it's my primary one
15mm Fish works PERFECT for WA shots, with enough room behind Dome for a 1.4x TC
For some reason, I get nicer bokeh/DOF on a FF camera
Superior LCD display useful when checking for backscatter
Cons:
Inability to use Tokina 10-17 (inflexible focal length)
More expensive
Not as tight when shooting the 100mm macro
Low light focus poor (countered by SOLA focus light)
Having said the above, I did not really take into consideration the Low light capability since I'm using strobes for most part. Also, I didn't think that the IQ difference between a Prime (Sigma Fish) and Zoom (Tokina 10-17) mattered much since I'm more concerned with getting good comps, getting little to no backscatter and also keeping myself alive. =) My choice in housing the 5D2 is mainly for blowing up prints, cropping capabilities and usability since I cannot make a good U/W image in-camera to save my life!