camera advice
 

I am looking for a camera for diving, and am trying to decide between 2 options. I have a Nikon D 70, with a 400mm lens. A protective housing for the camera is $1,400, and an additional housing for the lens is around $400. Another option would be a Nikon Coolpix with 6 megapixels, and a 3x optical zoom. This is $431. 1st, is there a difference in quality between the 2 cameras in terms of underwater pictures? Is it significant enough to justify the $1,400 difference? The Nikon D 70 housing fits in to my budget, but I would rather spend the $1,400 on my aquarium, as long as there is no significant difference in quality.

400mm is a big lens for underwater if you are referring to the 2.8 prime. Difference between a point & shoot and a DSLR is the size of the pixels. You get a larger pixel with a DSLR than a P&S because the sensor is bigger on a DSLR. Depending on how big you want to blow up a photo, I think a underwater P&S would be sufficient. I have a Nikon D300 and some big lenses but when I go to places where I am doing stuff and want to just document I carry my Canon IS850 which takes great photos that can be blown up to 11 x 14. If that is all you are looking to do I would save the money.

400mm lens would be VERY difficult to control under water, I'd think. Any movement at all would become camera shake and probably ruin more shots than it would be worth. I'd probably stick to something wider (105) and use the pixels to your advantage by cropping.

Personally I'd rather have my D200 & 105mm lens with me under water, but the coolpix would probably suit my needs for what I'd do with the photos afterwards (snapshots for sharing v. photographs for framing).

Cheaper to shoot film and pickup a used Nikos which is made for underwater.

If I do not use the 400 mm lens, then which option would you recommend? Is the D 70 better for taking underwater pictures then the Coolpix?

As you've all stated dslr's for underwater photography are very expensive. I would go with a good point and shoot that has an underwater case available, unless you're shooting professionally. I'm not sure if Nikon makes underwater cases, but Ikelite does for most makes and models. Also, Canon makes them for a lot of their point and shoots which are a lot less expensive than the Ikelite cases.
With all that said a member of my local club goes diving with a D200 with the 105mm lens and always has awsome pics to share with us.