My name's Lee Burghard, I'm an underwater videographer and co-owner of Wild Shutter Imaging(the link KC_moses had shown in the previous post). I'm also an equipment specialist for Reef Photo & Video and we carry Nauticam along with several other manufacturers.
I'm out filming/diving on regular basis, and because of my position at Reef Photo & Video I've been fortunate to have the opportunity try out a variety of cameras for U/W video shooting. I've spent quite a bit of time shooting with the Canon 6D/Nauticam combo over the last 2 years and would be happy to offer some insight on this setup based on my own personal experience.
Here's a couple of video samples of the Canon 6D/Nauticam combo in action:
We're fortunate that in today's U/W imaging market there are a variety of camera's available that will all do a very respectable job at taking U/W video. What I always tell people is that it's about finding the camera that compliments your specific needs, shooting style, and budget. I don't believe there's any one perfect camera for everyone, but I do believe that's it's possible to find the right camera for the individual.
The only minor fault I've found with the Canon 6D is it's ability to handle continuous auto-focus. This is typically true for most DSLR's or other larger sensor cameras shooting video. Unlike more traditional style camcorders with small sensors, the Canon 6D does not handle continuous auto-focus very well. When using continuous auto-focus while recording you'll catch/see the lense "hunting" when its tracking moving subjects, which doesn't look so wonderful when you're filming a giant whale shark cruising by
The method I've found that has worked best for me is setting up and locking in my focus before each shot. For wide-angle subjects that are not constantly moving closer or further away( maybe a sleeping turtle on a coral reef for example) from the lens this is an ideal approach. When I have wide-angle subjects that are constantly moving closer or further from the lens( lets say sharks swimming around as an example), I'll preset my focus at a fixed distance ( I like to focus off the bottom of my fin, so maybe 1.5-2 feet away) and closing my aperture/iris down( maybe f9 or more using either a fisheye or rectilinear wide-angle lens) to provide a greater depth of field. Using this approach essentially means that any subject from my focus point, 1.5-2 feet away from the lens, and beyond will pretty much be in focus.
Here's a few things that I really love about Canon 6D and Nauticam Housing for U/W filmmaking.
Excellent color through custom/manual white balance( this is true for every Canon DSLR I've used) - Out of every camera I've used, the Canon DSLR's handle white balance the best. I prefer shooting in ambient light as much as possible, and will only use video lights when I have to, so for me this feature is crucial. On a sunny day with good conditions I can get an accurate white balance using ambient light at depths of 80-90 feet with no difficulty( the goliath grouper video link I attached above was all shot with ambient light at depths between 70-90 feet).
A great range of lens choices - you have a variety of great lens to choose from to suit your subjects...everything from fisheyes( like the Sigma 15mm or Canon 8-15mm) rectilinear zooms ( Canon 11-24mm or 16-35mm) and macro lenses( like the Canon 100mm L). Choose your weapon!
Canon 6D has a large 35mm full-frame sensor, providing very good low light sensitivity. Simply put, on those dives where it's cloudy or you're deeper, and you need to bump up your ISO, that sensor will handle noise significantly better than other smaller sensor cameras.
Excellent ergonomic control on the Nauticam housing - this is a big one for me. When we're dealing with U/W wildlife, behaviors and moments happen fast! I shoot all of my cameras in full-manual mode so I need to make quick adjustments on the fly to things like aperture, ISO, shutter speed, and etc. I've used other housings before in the past, and one thing I've always liked about Nauticam is the very clever engineering involved to provide effortless changes of these settings. I can adjust all of my essential color and exposure settings on the housing without having to take my hands off the handles.
Ease of setup with the Nauticam housing - I can setup the whole underwater rig in minutes and I can swap out a battery, SD card, or change out a port/lens within seconds. When I'm doing 4-5 dives in a day for a shoot, this is important.
The Canon 6D( and other Canon DSLR's for that matter) are a budget filmmakers dream. They come in at a very cost efficient price for shooting cinema quality HD video, and provide you a lot of bang for your buck in my opinion. More importantly they won't cost you the price of a new car like the majority of high end cinema cameras will, and if you know how to use them properly they're capable of doing just as good of a job.
Getting the shot right the first time around - For me I don't always have the time ( or sometimes the want!) to spend hours doing color correction or exposure correction. Most of my projects and film shoots the client wants a finished/polished video quickly and efficiently. I do the majority of the editing, and don't have a dedicated colorist on payroll. So for me it's important that I nail the color and exposure correctly the first time around, and not have to do a bunch of post-processing work to get it right. That's the style and approach I prefer to go with and I can honestly say that the Canon 6D and Nauticam has allowed me to do that very successfully.
I could go on about what I like but that pretty much nails all the highlights! I hope all of that helps out and I didn't overload you with too much information. Forums like Wetpixel can be great sources for getting information and insight from other shooters. Please keep in mind that everything I've stated above is just based on my own personal experience using the gear you mentioned. If you have any questions feel free to send me a PM anytime, happy to share what has worked well for me with my U/W video shooting over the years.