Dark water diving initially to me was night diving. I have done a few dives in semi clear water with the worst being a few feet of visibility. Recently we did a training dive at Bear Lake and I got to experience what dark water diving was like. It was a sunny day, as we descended with our lights. At about 20' it was almost pitch black and I needed a light to see anything. As we went further down on the anchor line visibility went to near 0. I could not see my hand or anything else for that matter. Even with my trusty dive light there was nothing really clear to see. To make out anything I needed the light and for the object to be only a few inches away from my face. I had no idea where my dive buddy was and other than by feel, had no idea on my surroundings. I just hung out at the bottom of the water getting acclimated to both the cold temp and complete lack of visibility.
Our second dive was to practice actually recovering something. This dive was from a sandy bank on the lake. Once crossing the 20' depth all search efforts and maintaining a buddy was by feel. We did a few wiper motions and learned really quick about how to "sense" a buddy by rope feel. At 30' I certainly could not see him.
This being really the first dark water I've been in it was a different experience. Very calm diving like diving at night. It does require some modifications to the buddy system such as tug signals and anything done is by feel. Buoyancy is also very interesting. In deep depth a line or careful "Instrument" only flying was required. I had never used my computer light and depth gauge for instrument only buoyancy control and it took me a few minutes to get stable. Defiantly something to practice on for future dives.