Drew

Topside is much easier to shoot beautiful video with most cameras, but full frame is much more difficult to shoot underwater. I find the Nikon cameras have more noise compared to the Canons (with the exception of the D4, which has other video issues).

Depending on what he shoots, the D600 is a fine combo camera, but for video noise, it loses out to the Canon 6D. For video, the Canons have a bigger developmental base such as color profiles

It really depends on what he wants to shoot and how much he wants to spend.

If one can afford it, it's a no brainer, especially if there's a mixture of topside and underwater shooting.

Fixing it at the local Canon dealer would mean at minimum a 48 wait period (if you are a CPS member) and probably a week or more.  If it's a major drowning, they can't fix it anyhow.

I always have a backup camera which acts as a topside camera.  The one time I did a "Mustard" and went with one body to Bali because I already had a huge video rig. My friend drowned my camera just 3 days before my super long boat trips. I had to fly to the closest big country (SIngapore) to buy a replacement, only to find they had no stock available and I had to scour the 2nd hand shops and pay a premium for a body which was older and because of the country's price differential, cost 90% of a new one back in the US.
Of course, on those trips, I didn't like any of the pics I took except for maybe 3.  Was it worth all that trouble and expense when all I had to do was pack an extra body? NO!

Moral of the story, always have a backup camera.  If you don't want to spend on a DSLR, then get a smaller P&S like the Sony RX100 and housing.  It's more of a bonanza if you have optical strobes as it can be compatible.

It'll probably be like the D800 vs D800e, but a bit more difference in resolution.

Well, none of your RAW clips will look like that because he removed the OLPF/AA and ICF assembly.  That's why you see the IR shift in the clips.  The resolution is awesome though and the RAW just allows all that resolution to come out, with a bit of AA to boot.  I did that with my 1DsII a while back and the resolution made me keep the camera for a much longer time until it died.

MaxMax still does the conversions if you want to maximize resolution.

Simon, if you have the time, can you post the vectorscope and waveforms of the original AVCHD file for the clips with the green/cyan cast?  I'd like to see how much clipping there is in the G channel.  I think that cyan/green cast can be removed from the clips with a good grader like Resolve.