Yes I shoot ambient light in Socorro with Magic Filter. I know it's deep so I need to use custom white balance method to bring out the blue color. Unfortunately the visibility got bad half way into the trip so the white balance was challenging. At 3:30 into the video, you see some light on the Manta, it's a Keldan light from the boat's videographer, he swam around and shine his light everywhere and piss many people off, so watch out for people who abuse high power video light. Since I shoot ambient light, it's hard to swim on top of the Manta to capture the top side view of the manta because everything will be very dark. So it's a tricky experience for me.
As for your question of Canon 5DM4, I know 2 very skill videographers who are highly disappointed with it and decided to move to the 1DX M2 since the price is still manageable for them. Besides the file size, the big drawback of the 5DM4 is its crop factor, it's so bad that you can't really shoot good Wide Angle with it. I'm not sure if they corrected the crop factor through firmware update. Also, the 1DX M2 can shoot 4K 60 fps, so it's a highly desire feature with many professional videographers.