When I first started looking, I was gonna buy the D70. But then Canon released the 20D and I found myself in the same dilemma you are currently in. I'm an dyed in the wool amateur photographer and a newbie to dSLRs. After a bit of research, I made a semi-informed decision and I bought the 20D.
The big selling points for me were the higher MP of the Canon and the faster autofocus. I know that 8.2MP isn't that much more than 6.1MP from a quality standpoint. I reasoned that when I finally get this beast underwater with a huge housing and strobes and then I have to look through the viewfinder my marginal composition skills will rapidly approach none. Higher MP means I'll be able to crop some of the picture out and keep the same effective resolution. The autofocus is pretty self explanatory, but its going to help me get that shot when I've got a thousand things going through my little head. After reading more on the D70 vs. 20D debate, I too feel that the 20D is a better camera, but that's only my opinion.
As far as housings are concerned, the 20D has only been out since December 2004. The manufacturers with the more popular housings already have offerings for the 20D: Subal, Aquatica, Ikelite, etc. This spans the range from the high end to the value end of housings.
Just my $.02,
Ken