I too have the DC800 and was looking to do exactly the same thing.
I found a stay and arm set that works very well. Initially I found it difficult to know what adapter to use for the SeaLife Strobe. You need to keep that housing piece mounted on the strobe in order to attach the optical cable.
I have a light cannon and had been using it for video periodically without it being attached to the camera strobe. One hand holding the cannon and another the camera... what a pain.
So I finally found someone in my area (well, just south of me in Seattle) who stocked a bunch of light arms and adapters. He was kind enough to let me try a bunch and found one that worked for the SeaLife Strobe. It was the hotshoe ball adapter. I just needed to get a 1.25 inch stainless steel stud to connect the adapter to the Strobe.
He had the UK Cannon adapter as well. So I now have a true tray and Arm set with the cannon on the right (primary grip and camera trigger side because it is a bit heavier than the strobe) and the strobe on the left.
One of the good things was that I didn't have to pay the Ultralight prices. I have used it on several dives so far (just got it 1.5 weeks ago), and am thrilled with the stay and arms.
As far as the performance of the light cannon, it definately has a hot spot with one diffuser. I was actually browsing the board (when I came across this thread) looking for info on whether two diffusers would help. It seems so and I will be putting another one in for this weekend's dives.
I have a question though: When I bought the arm set, I purchased an 8", two 6", and a 2" arm. with the longer arms the whole thing gets a bit crazy looking so I have actually limited it to a ball-to-ball connection on the strobe side, and the two inch arm on the cannon side. Would I improve the image quality of the video by moving the light further away from the lens? I am thinking that it may because the light would be angling from the side rather than shooting through the particulate directly in front of the lens. (remember I am in the green waters of the pacific northwest and British Columbia).
I should also put a plug in for where I got the stay and arms (as I said i am very pleased). Check out: opticaloceansales.com ... I got the S-tray and double arm set.
Oh also... you asked about the bigblue lights. Jack carries those too and I asked about them. Certainly the small one is just a focus light. I think it produces enough light to allow the still camera to focus but not really a true video light. I can't comment intelligently about the bigger one at all. There is also the new UK eLED light that may be worth checking out. apparently 850 lumins compared to the 400 of the cannon.
