Here is main issue. Not sure if its a problem with the dc600 or not.
The DC500 needs quite a bit of light in order to focus. This means
that you have to shine the dive light at the subject to get it to focus,
then move it away to avoid a bright hot spot in the photo.
This is actually an issue with most any camera, but here is something to consider...
If you need a way to avoid this take a look at purchasing a focus light. This could be used to help focus durring the day in low light and close up shots or at night and you would not need to balance the dive light or worry about the hot spots....
The attached link is a very cost effective option and all you need is a hot shoe on you housing. You could purchase a focus light for more $$ but this should work fine for most cases.
Link:
Reef Photo & Video!, The Underwater Photo Pros
Link for hot shoe attachment:
Sea And Sea Hot Shoe, Underwater Cameras, Sea & Sea, Sea And Sea Hot Shoe
The attachment cost more than the light but both together are less than some dive lights.