The majority (all?) of the current crop of Evils are 4/3rds cameras and they have only been out for a few years. As a result of that, the number of lenses is not as great as the number of lenses available for a Canon or Nikon -- but really, how many do you need? (The answer -- more than you have!!!!)
Yes, there appear to be fewer housing options for the Evils (the Pen EPL-x for example) but remember, there may not be that many housings for any particular model of a Canon or Nikon either and it is unlikely that any one housing will fit more than one model.
One issue for the Evil vs. a dSLR -- are you going to use Live View or use the View Finder while shooting UW? I have very limited experience with a dSLR underwater but I found using a View Finder to be almost worthless. That was ONE of the reasons I opted for the Pen when I finally upgraded from a PnS. I don't use manual focus (my eyes are not that good!) and I wanted to use a "live view" system for composing my image and the Evil (Pen) was perfect for what I wanted:
a. Interchangeable lens when/if I wanted to spend the money;
b. "Real" sensor -- yes the 4/3rds are smaller than my Canon but still a huge step up from a PnS;
c. Smaller and with the housing, significantly less mass (weight) than a dSLR -- and my Pen with the Oly housing and Inon strobe is very well balanced UW.
For me, the Pen was the right choice -- more sophisticated than a PnS -- lighter, smaller and less expensive (with the housing, much less expensive) than a dSLR.