I shoot Canon. However, a few years ago, when I was DSLR Shopping, I was torn between Canon and Nikon. I decided on Canon, because I found a great deal on a used Canon RebelXt. However, in researching, Canon/Nikon, this was about the time the Nikon D70 was introduced, I came across a lot of information, that led me to the opinion that the Nikon D40 was actually a better camera, than the D60, and both were better than the D70.
That being said, a Nikon D70, in good condition, along with decent lenses, will serve your purposes very well, just as the Canon RebelXt served me well, for several years. In our type of photography (reef photography) the camera body is actually a secondary consideration. The lens is where it's at, AND where you'll spend the most money. Any Nikon or Canon dSLR, in good working order, will take great photos, with the proper lenses and the proper knowledge of the photographer.
As far as "ease of use," having never owned a dSLR, you're going to be in for a surprise. You'll have a learning curve that you'll need to overcome. Ignore the "owners manuals," as they're pretty useless. Get yourself a couple good books and join a couple good photography forums. Aside from that, there's great photography tips available, here on R2R. When you first get a dSLR, concentrate on learning what all the buttons do and how to change all the settings. Eventually, you'll want to learn to shoot, mainly in Manual Mode or Aperture Priority Mode. These terms will become more clear to you, over time.