The only really good entry level SLRs are the canon ones like ts2i. They have a million housings and can take the full set of lenses, including the tokina 10-17, which is the ideal wide angle lens for many uw photogs.
I am perfectly happy with ikelite housings, and been using many different ones since the early 90s. I see no reason to ever spend more money, unless you are trying to go pro, or shoot underwater many months per year. The ikelite is half the price of the alu housings.
The nikon entry SLRs do not have a focus motor, so they cannot do auto focus with many of the lenses that you would want to use, including the tokina. You have to go up to a d7000 to get a really functional DSLR.
Oly's SLRs are not up to snuff compared with canon or nikon.
I agree with Steve, that at this point, a good interchangeable lens, mirrorless camera makes much more sense, for many folks. The micro 4/3 cams have sensors very close the the sensor in the DSLRs, at much less weight. They are made by both oly and panasonic, and have vastly more lens offerings than the sony NEX family.
The oly micro 4/3 cameras have lots going for them. And oly makes quite reasonable price housings for many of their cameras.
My current dream camera is the oly omd em5, which has the latest and greatest sensor for this size, which is fully competitive with all the APSC DSLRs for image quality, and dynamic range. Fantastic auto focus, and a built in electronic view finder. The oly housing is due out this summer.
You can read about it at dpreview and wetpixel.