Like Darren said, you're looking at thousands of dollars for a decent underwater housing. The plastic bag types will only last a few dives, and a single grain of sand in the seal will be the end of your camera.
More food for thought: housings are camera-specific. If you buy one for your D70 it is unlikely you can use it for a future camera, so make sure the D70 is going to be your underwater camera for a long time before laying out tons of cash.
A friend of mine is a divemaster and underwater photographer. We're talking about getting a housing for my D2x and he warned me to get a D200 housing (which would also mean buying a D200!) because he says: "there are two kinds of underwater photographers - ones who have drowned their camera, and one who are going to drown their camera."
In the interim, I bought a P-n-S camera with an underwater housing that is good to 100 feet. It can't take external strobes (which really are necessary if you go down more than 10 or 15 feet, or if it's cloudy up above) but it's great for just goofing around and getting vacation pic's. Price of the camera and housing together were 1/6 the cost of a "cheap" housing for my D2x. All the underwater pictures on this site were taken with the point-n-shoot: http://www.bulek.com/cozumel.htm