What settings do you have your camera on?
I have a DC500 with an external strobe as well. When diving I have it on the fish mode and then advance it to a shark mode (press the ok button). The only pictures I have with it thus far are from Nova Scotia and Aruba (both with no strobe) and the conditions were extremely different. In Nova Scotia the water was silty and had a lot of suspended particles, the weather was overcast and at times raining. My shots, even with out the strobe came out fairly well. There is a bit of sediment and suspended junk but otherwise they are very clear and not dark (that was at 55 feet). In Aruba I was diving between 90 and 100 feet (I had the strobe, thought it was working... but the batteries in upside down on the plane and forgot to change them) The shots are a little darker but still quite clear. Because of the depth ofcourse the color is washed out a bit from farther away but I to superstructre of wrecks etc no problem... my only problem there was the suspended junk again... *note to self... change batteries.... *
Have you been using the shark mode? If not that might be part of it. I found the first time using it above water that I almost had to hold my breath to get a good shot becuse of the delay between the press of the button and the actual taking of the picture. Underwater my first set came out as you said - dark superstructure and ... well... generally dark.
I plan on diving in the next few weeks, as soon as the weather improves a bit here, and plan on taking my camera with me. I will play around with some more of the settings and see what I get. I'll not which pictures I took on which dive, what the profile was and what setting my camera was on and go from there.
It seems to me, if I am not mistaken... that some of my best shots have actually been on the 'auto' mode (this time with the strobe) but again, I play around with it.
Let me know if you find anything out (I'm from Nova Scotia, know all about the dark water deal ... if I;m not there, I'm in Calgary...

). I haven't changed the settin on my camera since my last dive in Nova Scotia so I will start with that. Waters here (Aus) are of course going to be a fair bit lighter than what we are both used to, but a little experimentation won't hurt!
Best of luck
AP