personally I find those colours absolutely awesome! I'll definitely try experimenting with a grey card next time out but, as you probably know, I'm actually really happy with the MWB on my GH5 (red filter/grey translucent plastic). Using ambient light like in your video gives everything a much more natural feel, IMHO, plus you avoid having to deal with backscatter and annoying the marine life with thousands of lumens in their eyes. At around 2'50" you were filming inside the wreck, and I'm pretty sure the GH5 wouldn't have handled that anything like as well.
GH5 + Nauticam. WB really does work, and 4K 50/60 is just great to have. 10-bit 1080 50p, 10-bit 4K 25p, 1080 up to 180 fps, 4K up to 1.... someone on here said they could not really think of what more to ask for, and I agree. In practice DoF depends on aperture surely? I use the PZ 14-42 3.5-5.6, and DoF isn't really an issue. I guess with your +5 diopter you would have to be careful, but one of the great advantages of 4K is that you can crop and still end up with a full HD image, so you don't always need to be fully zoomed in underwater. Also, 50p slowed down to 25p in post looks very smooth. I don't do macro, but the best macro stuff you see on vimeo etc seems to have been filmed with a tripod. Don't forget that you'll need a decent computer and good NLE to edit!