Steven,
I bought one of the Simms roll-top fanny packs just for that purpose. It has a padded interior that cradles the D70 pretty nicely, although it doesn’t leave much room for anything else. It will accommodate the camera with a fairly large lens (holds my 80-200), or it will hold the camera and two smaller lenses. I would never wade in deep water with the bag, but I’m fairly certain it will survive a quick dunking.
A guide friend has a Sagebrush Dry Goods Fanny Pack that Buzz mentioned. It has a water -proof zipper. He uses it in the salt on bone fishing trips and told me he has even snorkeled in it. I thought they were out of business, but if not, that definately would be worth looking at.
If you are looking for a boat bag, Simms and Patagonia both make excellent dry bags. I bought the Simms bag, but now wish I had opted for Patagonia’s Great Divider. The Simms Bag is just too big and I have had some trouble getting it under some airline seats and in overhead bins. However, I think Simms is/has come out with some smaller versions of the bag.
Hope this helps.