dscribner, wiring refresher ...
Basically, most of us (with a combo steaming/deck light) will have an all-around anchor light at the top of the mast and the combo at mid-mast. Yes, the three lights should work independantly. The all-round at the top is on just when anchored, the steaming light is on with the bow lights and stern light when motoring only - not sailing at night. When sailing with bow lights and stern light on, the steamer is off. The deck light is for utility only, and is most likely going to be used when anchored or moored. It will ruin you night vision if you have it on while sailing or motoring so it is not a good idea to have it on then. I suppose that if you are in a congested area and there is some trepidation that you are not being seen, then it could be used to warn others that you are in the area and wish to be seen, but I wouldn't recommend it for that general purpose.
All that said, I wanted to simplify my wiring to some extent so that I could use a four prong plug at the mast connection. Obviously I have 3 positive leads going back to my switches. I will acknowledge that it is preferable to have a separate negative for each positive going back to the negative buss, but that would mean a 6 prong plug. Besides, if I'm not mistaken, the combo (mine is Hella) has an internally spliced negative lead. For positives, I have 2 - 16 guage wires (for the anchor and steamer) and a 14 guage for the deck light because of the length and the 20 watts. I have a 16 guage negative coming down from the anchor and a 14 guage negative coming down from the combo. Above the plug, I splice the 2 negatives to a single 12 guage negative. That way I have 3 pos and 1 neg for the 4 prongs. The 12 guage negative is all that runs back to the buss. I feel comfortable with this primarily because there is no need to have the 3 lights on at one time. I normally use the combo only when I'm on the mooring anyway so I typically have only one of the lights on at a time.
When I'm anchoring, I just use my 10,000 candle power spotlight to annoy everybody else around us! (Just Kidding!)